Commitments API: push or pull?

[mempirate]

Context

The current version of the commitments API is a push-based API: users (or the Bolt RPC) send requests directly to the sidecar. This has the undesirable side effect of the sidecar having to be exposed to the internet. mev-boost does not have this - everything is pull based.

Should we think about making it configurable? I.e. proposers can specify a pull-based RPC, where Bolt RPC will buffer up requests and pack them into a response when the proposer polls it. This will add latency but is much safer for the proposer.

Obviously we also retain the option of exposing it directly for the more sophisticated node operators.