Batch signing endpoint

[eth2353]

The remote signing API currently only supports signing a single piece of data for a specified BLS key. When running a large number of validators this means sending thousands of HTTP requests to web3signer, meaning a lot of unnecessary overhead resulting in a slowdown of validator duties. When processing each of those requests web3signer also needs to compute the exact same signing root thousands of times, wasting further CPU cycles.

It would be great if the remote signing API specified a way to request signatures over the same data for multiple keys at once. While this is not applicable to all message types, it can be used for many of them (e.g. attestations especially now with EIP-7549).

With conversations taking place about shortening slot times, this kind of efficiency improvement on the remote signing API side seems as relevant as ever.

For context, while it does not adhere to the Remote signing API, the remote signer Dirk supports these kinds of batched signing requests.

[aimxhaisse]

Ideally it'd be optional so that existing architectures that depend on the former can still operate smoothly.

Having per-request granularity allows an ingress to spread the load evenly, scale-up dynamically replicas, and also allows smooth upgrades (i.e: redirect 1% of traffic to a new version for instance, before going to higher levels).