Standardize Warp Signature Requests across VMs

[cam-schultz]

Warp message signatures consist of an aggregate BLS signature composed of the individual signatures of a subset of a subnet's validators. Validator signatures should be retreivable by any party that wishes to construct an aggregate signature over a Warp message. At present, this is left to VMs to implement, as is the case with Subnet EVM and Coreth

This creates friction in applications that are intended to operate across many VMs (or distinct implementations of the same VM). As an example, the reference Warp message relayer implementation, awm-relayer, fetches individual signatures from validators and aggregates them before sending the Warp message to its destination chain for verification. However, Subnet EVM and Coreth have distinct codecs, requiring the relayer to switch according to the target codebase.

Another example is ACP-75, which aims to implement acceptance proofs using Warp. The signature aggregation mechanism is not specified, which is a blocker for that ACP to be marked implementable.

Standardizing the Warp Signature Request format by implementing it as a protocol-level (AvalancheGo) AppRequest type would simplify the implementation of ACP-75, and streamline signature aggregation for out-of-protocol services such as Warp message relayers.