feat: add support for pallet-xcm claimAssets call

[marshacb]

Description

This PR adds support for constructing local claimAssets runtime calls to the AssetTransferApi for claiming trapped assets.

Claimable Assets

Assets to claim can be provided as asset symbols (e.g. usdt, wnd), locations (e.g. {"parents":"0","interior":{"X2":[{"PalletInstance":"50"},{"GeneralIndex":"66"}]}}) or integers (e.g. 66, 1984).

Note: An asset can also be provided as an empty string. In this case the assetId will be recognized as the relay chains native asset. (e.g. [''] will be equivalent to wnd when on a Westend chain).

Example of constructing a claimAssets call to claim trapped WND on Westend:

api.claimAssets(
	['WND'],
	['1000000000000'],
	'5HBuLJz9LdkUNseUEL6DLeVkx2bqEi6pQr8Ea7fS4bzx7i7E', 
	{
		xcmVersion: 4,
		format: 'payload',
		sendersAddr: '5EWNeodpcQ6iYibJ3jmWVe85nsok1EDG8Kk3aFg8ZzpfY1qX'
	} 
)

Expected output:

{"origin":"0","dest":"westend","direction":"local","xcmVersion":4,"method":"claimAssets","format":"payload","tx":"0xc8630c0404000000070010a5d4e80400010100e2aa84cbfa479f0150bb77eb06756ee2c725ec625e16dd68b2678a0820e8d834b500940050690f0018000000e143f23803ac50e8f6f8e62695d1ce9e4e1d68aa36c1cd2cfd15340213f3423e901c8a5932603948278d24d86dfc72e508d890723bf6bd3cf9f8645be53cbdc2"}

closes: #373

[TarikGul]

LGTM, great job. Just a a few small nits in the examples

[Polkadot-Forum]

This pull request has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/paritytech-update-for-april/7646/1