Implement Ecdsa signature verification

[radkomih]

There is a TODO in the codebase:

let m = sp_io::hashing::blake2_256(msg.get());
match sp_io::crypto::secp256k1_ecdsa_recover_compressed(sig.as_ref(), &m) {
 	Ok(pubkey) =>
 		&sp_io::hashing::blake2_256(pubkey.as_ref()) ==
 			<dyn AsRef<[u8; 32]>>::as_ref(who),
 	_ => false,
}