[Identity] Remove double encoding username signature payload (#4646)
In order to receive a username in `pallet-identity`, users have to, among other things, provide a signature of the desired username. Right now, there is an [extra encoding step](https://github.com/paritytech/polkadot-sdk/blob/4ab078d6 /substrate/frame/identity/src/lib.rs#L1119) when generating the payload to sign. Encoding a `Vec` adds extra bytes related to the length, which changes the payload. This is unnecessary and confusing as users expect the payload to sign to be just the username bytes. This PR fixes this issue by validating the signature directly against the username bytes. --------- Signed-off-by:georgepisaltu <[email protected]>