Deprecate Warp API in favor of Signature Aggregator API

[cam-schultz]

Context and scope
The relayer can aggregate ICM signatures directly via AppRequest, or indirectly by calling Subnet EVM's signature aggregation API. The signature-aggregator service in this repository performs the same function as Subnet EVM's API, in that it abstracts the p2p signature collection and aggregation behind a remote service. Both are useful for cases in which a relayer operator does not want to connect over p2p to the wider Avalanche network.

Discussion and alternatives
We should use signature-aggregator instead of the Subnet EVM API for a few reasons:

• Reduces external dependencies
• Opportunity to simplify config (e.g. the config option could be externalSignatureAggregator which when omitted implies that the relayer itself aggregates signatures. As is, it's not clear that the Warp API config option is optional)
• Use a more battle-tested service
• The Subnet EVM aggregation API may be deprecated in the future in favor of a unified aggregator implemented in Avalanchego

Note that the Warp API integration should still be preserved as part of this change in order to provide ample deprecation notice. The new external signature aggregator option should be incorporated like so:

if cfg.WarpAPI is set {
    // Use Warp API
} else if cfg.ExternalSignatureAggregator is set {
    // Use external signature-aggregator service
} else {
    // Aggregate signatures over P2P
}