[REQ] Support for google.api.http in protobuf-schema generator

Is your feature request related to a problem? Please describe.

When I generate protobuf-schema schema for gRPC I want also to be able to generate for REST also.
Here is explanation https://cloud.google.com/endpoints/docs/grpc/transcoding

For example like this:

// Returns a specific bookstore shelf.
rpc GetShelf(GetShelfRequest) returns (Shelf) {
// Client example - returns the first shelf:
// curl http://DOMAIN_NAME/v1/shelves/1
option (google.api.http) = { get: "/v1/shelves/{shelf}" };
}

Describe the solution you'd like

To provide a parameter to turn on/off the feature.

Additional context

I wanted to do by myself and I think is here (line 19)
https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/protobuf-schema/api.mustache
but I don't know parameters.

Start with modules/openapi-generator/src/main/resources/protobuf-schema/api.mustache, especially the referenced area around line 19, and review the linked gRPC transcoding explanation. Determine how a parameter could enable or disable google.api.http handling. Done means protobuf-schema generation supports the REST mapping example when enabled while preserving the existing gRPC output when disabled.