add support for "auto" as kubectl version with use subcommand

Signed-off-by: Navratan Lal Gupta <navilg0409@gmail.com>

README.md

@@ -77,7 +77,7 @@ k8senv [VERB] [CLIENT] [VERSION]
 
 # [VERB] can be install, use, list or remove
 # [CLIENT] can be kubectl, velero or helm
-# [VERSION] can be any client's version in format v*.*.* or latest or auto (only supported for kubectl and velero)
+# [VERSION] can be any client's version in format v*.*.* or latest or auto (auto is only supported for kubectl install and use subcommands)
 
 examples:
 
@@ -120,6 +120,7 @@ Any of below commands can be used to switch kubectl version to 1.26.2. If versio
 k8senv use kubectl v1.26.2
 k8senv use kubectl 1.26.2
 k8senv kubectl use v1.26.2
+k8senv use kubectl auto
 ```
 
 If you were using system installed version of `kubectl` in same shell session, you will need to clear the cached path of the kubectl using below command.

internal/use/use.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"github.com/navilg/k8senv/internal/config"
+	"github.com/navilg/k8senv/internal/ikubernetes"
 	"github.com/navilg/k8senv/internal/install"
 )
 
@@ -19,6 +20,17 @@ func UseVersion(toolname, version string) error {
 		return errors.New(".k8senv/bin is not added in PATH environment variable")
 	}
 
+	if toolname == "kubectl" && version == "auto" {
+		fmt.Println("Fetching Kubernetes server version")
+		k8sVersion, err := ikubernetes.GetK8sVersion()
+		if err != nil {
+			fmt.Println("Error getting Kubernetes server version")
+			return err
+		}
+		version = *k8sVersion
+		fmt.Println("Kubernetes server version is", version)
+	}
+
 	major_minor_patch_vers := strings.Split(version, ".")
 
 	if !strings.HasPrefix(major_minor_patch_vers[0], "v") {