[XCM] add generic location to account converter for locations in external ecosystems

[acatangiu]

We need a location to account converter that can be used by any parachain in Polkadot/Kusama and it can convert any location from an external GlobalConsensus.

We basically want a converter that encompasses GlobalConsensusConvertsFor and GlobalConsensusParachainConvertsFor, but also enhances them with DescribeFamily-like support.

The implementation is simply a concatenation of them.

[Nathy-bajo]

Working on this @acatangiu

[acatangiu]

Ok, let me know if you need any help 👍

[Nathy-bajo]

Hi @acatangiu I tried attempting this.
Please review, I'm writing a test for the fix.

[acatangiu]

Ideally we want a single new convertor that can replace these 3 convertors in a fully backwards compatible way. Meaning any AccountId->Location conversion currently covered by these 3 will also be covered by the new one and provide the exact same output/conversion.

Example tests here that can be enhanced to ensure backwards compatibility.

On top of covering the existing set of Locations as the 3 convertors mentioned above, it should also cover external global consensus locations.

For example, GlobalConsensusParachainConvertsFor currently converts Location { parents: 2, interior: X2([GlobalConsensus(bla), Parachain(X)]), but does not support child locations of it, such as Location { parents: 2, interior: X3([GlobalConsensus(bla), Parachain(X), AccountId(Alice)]).

One way is to create a new tuple that starts with existing converters and ends with a new one for the new set of covered locations:

pub type GlobalLocationToAccount<UniversalLocation, AccountId> = (
	// Foreign locations alias into accounts according to a hash of their standard description.
	HashedDescription<AccountId, DescribeFamily<DescribeAllTerminal>>,
	// Different global consensus parachain sovereign account.
	// (Used for over-bridge transfers and reserve processing)
	GlobalConsensusParachainConvertsFor<UniversalLocation, AccountId>,
	// Ethereum contract sovereign account.
	// (Used to convert ethereum contract locations to sovereign accounts)
	EthereumLocationsConverterFor<AccountId>,
	// Used to convert any other external global consensus locations to sovereign accounts.
	ExternalConsensusLocationsConverterFor<UniversalLocation, AccountId>,
);

ExternalConsensusLocationsConverterFor should:

• validates the location is from different consensus
• derives some string/bytes from that external consensus (smth like GlobalConsensusParachainConvertsFor)
• derives some string/bytes from the sublocation remaining after processing above (smth like DescribeAllTerminal)
• concatenates and hashes above for a final derived AccountId

(also because EthereumLocationsConverterFor is already used and should be supported, but cannot be directly added to the tuple because it lives in a different downstream crate, you should just hardcode it inside ExternalConsensusLocationsConverterFor).