Add an example that fetches token balance of account (#509)

first commit to add example that fetches token balance of an account

examples/getTokenBalance/.env.template

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+# use this variable to change the RPC base URL
+#INTEGRATION_BASE=http_insert_end_point
+#ACCOUNT_ADDR=account_contract_address

examples/getTokenBalance/README.md

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+Note: To run this example, you need a mainnet endpoint.
+
+Steps to run this example on mainnet: 
+1. rename ".env.template" to ".env.mainnet"
+2. uncomment, and set INTEGRATION_BASE to the mainnet url
+3. uncomment, and set ACCOUNT_ADDR to the account contract address
+4. make sure you are in the "getTokenBalance" directory
+5. execute `go mod tidy`
+6. execute `go run main.go`

examples/getTokenBalance/go.mod

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+module account
+
+go 1.21
+
+require (
+	github.com/NethermindEth/juno v0.3.1
+	github.com/NethermindEth/starknet.go v0.2.1-0.20220620163912-1db2ca279608
+	github.com/ethereum/go-ethereum v1.10.26
+	github.com/joho/godotenv v1.4.0
+)
+
+replace github.com/NethermindEth/starknet.go => ../../
+
+require (
+	github.com/StackExchange/wmi v0.0.0-20180116203802-5d049714c4a6 // indirect
+	github.com/bits-and-blooms/bitset v1.7.0 // indirect
+	github.com/consensys/gnark-crypto v0.11.0 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/deckarep/golang-set v1.8.0 // indirect
+	github.com/fxamacker/cbor/v2 v2.4.0 // indirect
+	github.com/go-ole/go-ole v1.2.1 // indirect
+	github.com/go-stack/stack v1.8.0 // indirect
+	github.com/gorilla/websocket v1.4.2 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/shirou/gopsutil v3.21.4-0.20210419000835-c7a38de76ee5+incompatible // indirect
+	github.com/stretchr/testify v1.8.1 // indirect
+	github.com/test-go/testify v1.1.4 // indirect
+	github.com/tklauser/go-sysconf v0.3.5 // indirect
+	github.com/tklauser/numcpus v0.2.2 // indirect
+	github.com/x448/float16 v0.8.4 // indirect
+	golang.org/x/crypto v0.2.0 // indirect
+	golang.org/x/sys v0.3.0 // indirect
+	gopkg.in/natefinch/npipe.v2 v2.0.0-20160621034901-c1b8fa8bdcce // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

examples/getTokenBalance/go.work

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+go 1.21
+
+use (
+	.
+	../..
+)

examples/getTokenBalance/main.go

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+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"math/big"
+
+	"github.com/NethermindEth/juno/core/felt"
+	"github.com/NethermindEth/starknet.go/rpc"
+	"github.com/NethermindEth/starknet.go/utils"
+	ethrpc "github.com/ethereum/go-ethereum/rpc"
+	"github.com/joho/godotenv"
+)
+
+var (
+	name               string = "mainnet"
+	ethMainnetContract string = "0x049d36570d4e46f48e99674bd3fcc84644ddd6b96f7c741b1562b82f9e004dc7"
+	// usdcMainnetContract string="0x053c91253bc9682c04929ca02ed00b3e423f6710d2ee7e0d5ebb06f3ecf368a8"
+	// daiMainnetContract string="0x00da114221cb83fa859dbdb4c44beeaa0bb37c7537ad5ae66fe5e0efd20e6eb3"
+	getBalanceMethod  string = "balanceOf"
+	getDecimalsMethod string = "decimals"
+)
+
+// main entry point of the program.
+//
+// It initializes the environment and establishes a connection with the client.
+// It then calls a token contract to get the account balance and decimals.
+// In the end, it transfomers the response to readable number.
+//
+// Parameters:
+//
+//	none
+//
+// Returns:
+//
+//	none
+func main() {
+	fmt.Println("Starting getTokenBalance example")
+	godotenv.Load(fmt.Sprintf(".env.%s", name))
+	base := os.Getenv("INTEGRATION_BASE")
+	c, err := ethrpc.DialContext(context.Background(), base)
+	if err != nil {
+		fmt.Println("Failed to connect to the client, did you specify the url in the .env.mainnet?")
+		panic(err)
+	}
+	clientv02 := rpc.NewProvider(c)
+	fmt.Println("Established connection with the client")
+
+	tokenAddressInFelt, err := utils.HexToFelt(ethMainnetContract)
+	if err != nil {
+		fmt.Println("Failed to transform the token contract address, did you give the hex address?")
+		panic(err)
+	}
+
+	accountAddress := os.Getenv("ACCOUNT_ADDR")
+	accountAddressInFelt, err := utils.HexToFelt(accountAddress)
+	if err != nil {
+		fmt.Println("Failed to transform the account address, did you give the hex address?")
+		panic(err)
+	}
+
+	// Make read contract call
+	tx := rpc.FunctionCall{
+		ContractAddress:    tokenAddressInFelt,
+		EntryPointSelector: utils.GetSelectorFromNameFelt(getBalanceMethod),
+		Calldata:           []*felt.Felt{accountAddressInFelt},
+	}
+
+	fmt.Println("Making balanceOf() request")
+	callResp, err := clientv02.Call(context.Background(), tx, rpc.BlockID{Tag: "latest"})
+	if err != nil {
+		panic(err.Error())
+	}
+
+	// Get token's decimals
+	getDecimalsTx := rpc.FunctionCall{
+		ContractAddress:    tokenAddressInFelt,
+		EntryPointSelector: utils.GetSelectorFromNameFelt(getDecimalsMethod),
+	}
+	getDecimalsResp, err := clientv02.Call(context.Background(), getDecimalsTx, rpc.BlockID{Tag: "latest"})
+	if err != nil {
+		panic(err)
+	}
+
+	floatValue := new(big.Float).SetInt(utils.FeltToBigInt(callResp[0]))
+	floatValue.Quo(floatValue, new(big.Float).SetInt(new(big.Int).Exp(big.NewInt(10), utils.FeltToBigInt(getDecimalsResp[0]), nil)))
+
+	fmt.Printf("Token balance of %s is %f", accountAddress, floatValue)
+}