signMessage(...): move encoding and hashing logic to client side

[keyz-tk]

Summary & Motivation

For raw payload signing, our configuration is too inflexible -- currently we require the message to be UTF8 encoded, and we hash the payload via KECCAK256 on the remote signer.

This PR changes signMessage(...) to conform to what Ethers expects, eth_sign style:

• The method accepts either a UTF8-message (string) or a binary message (Bytes)
• Hashing is done in JS land and conforms to EIP-191 (see https://docs.ethers.org/v5/api/utils/hashing/#utils-hashMessage)
• Remote signer always receive hex-encoded payload (PAYLOAD_ENCODING_HEXADECIMAL), and does not apply extra hashing (HASH_FUNCTION_NO_OP)

How I Tested These Changes

CI and running internal E2E tests manually

[r-n-o]

Neat!