[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/4ab078d6754147ce731523292dd1882f8a7b5775/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.

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Signed-off-by: georgepisaltu <[email protected]>