ACP 118 reference implementation

[cam-schultz]

Why this should be merged

• Adds proto definitions for SignatureRequest and SignatureResponse AppRequest/AppResponse message types as defined in ACP-118
• Defines the canonical handler ID for SignatureRequests as 1, as specified in ACP-118: Specify SignatureRequest handler ID avalanche-foundation/ACPs#129
• Defines the canonical handler ID for SignatureRequests as 2, as specified in ACP 118 - update handler ID and rename field avalanche-foundation/ACPs#132
• Moves the existing definition of the handler ID for Tx Gossip messages to the p2p package

Does not go so far as to add a new package under network/p2p with message parsing, message marshalling, default handlers, etc. ala the gossip package.

How this works

See above

How this was tested

CI

[michaelkaplan13]

Makes sense to me, would defer to those with more context of the codebase to comment on where the definitions live within the files/packages.

[tsachiherman]

looks good to me, but would be happy to have others review this as well.

[StephenButtolph]

After adding the couple requested comments - lgtm