support manual recipient in xTokens

[zqhxuyuan]

current beneficiary in xTokens testcase are all kind of destination normal account, may be in some cases, the recipient isn't normal account on destination chain, but sovereign account. for ex: https://github.com/paritytech/polkadot/blob/release-v0.9.12/xcm/xcm-builder/tests/scenarios.rs#L269 in this case, paraA transfer relay token to paraB, but the final beneficiary is paraA sovereign account on paraB. in xTokens send_relay_chain_asset_to_sibling testcase https://github.com/open-web3-stack/open-runtime-module-library/blob/master/xtokens/src/tests.rs#L112, seems it doesn't support such case.