Move all rmb handlers to api-gateway

[AbdelrahmanElawady]

Description

Move all rmb handlers to api-gateway and use peer.Router instead of using rmb-peer binary.

Changes

• All defined handlers and endpoints are now found under pkg/api-gateway.
• Unimplemented endpoints reply immediately that it's not implemented.

Related Issues

• #2200

Checklist

• [ ] Tests included
• [x] Build pass
• [x] Documentation
• [x] Code format and docstring