[Feature Request] Support All Potential Types in EIP712 Signatures

[BrettCleary]

Currently only string fields are supported in an eip712 JSON data structure. Further, generating the message hash is very error-prone.

Functionality to generate a message hash from a generic eip712 JSON structure is needed.

The signing and recovering nodes do not need to be changed for this.

References:

• https://eips.ethereum.org/EIPS/eip-712
• https://medium.com/metamask/eip712-is-coming-what-to-expect-and-how-to-use-it-bb92fd1a7a26