Improvements on server/v2 gRPC gateway autoregistration

[julienrbrt]

Follow up of #22941

• Create a doc.go file in server/v2/api/grpcgateway explaining the syntax supported by the catch all handlers
• Remove manual grpc grpcgateway registration from modules: feat(server/v2)!: grpcgateway autoregistration #22941 (comment)
• Possibly keep manual grpc gateway support for third party modules by having catch-all handler be first/last (to verify) handler (so manual registration overrides catch-all handler)
• currently, regexp patterns are built on the fly, but we could build these for each URI beforehand, and store them for faster processing.