Deprecate and remove grpc-gateway (JSON responses to gRPC services)

[prestonvanloon]

🚀 Feature Request

Description

For several years, Prysm has used a fork of gRPC gateway with a replace statement in go.mod. The original context can be found in #8697. The gist of how that context relates to this issue is that the fork of grpc-gateway has some crude limited support for certain cast types in Prysm as well as some proto definitions that probably could be included here within the prysm repo. Additionally, there seems to be some changes with regards to event streaming.

See the diff between the upstream grpc-gateway and Prysm's fork. grpc-ecosystem/grpc-gateway@main...prysmaticlabs:grpc-gateway:master

Describe the solution you'd like

The Beacon APIs have become the standard interface for accessing JSON data from beacon nodes. As such, Prysm no longer needs to offer grpc-gateway JSON translations of gRPC request / responses after the completion of standard API support in Prysm.

• Announce deprecation plan at least 1 release prior to removal
• Remove usages of grpc-gateway entirely
• Remove dependency on Prysm's forked grpc-gateway
• Move still-relevant protos into prysm (proto/gateway/event_source.proto)
• Ensure no breaking changes with regards to the events API streaming service

Describe alternatives you've considered

Users can access JSON data via the standard beacon APIs. See: #12635