grpc-loader: Expose field options

### Is your feature request related to a problem? Please describe.
In our proto files we would like to use custom options on field level (see [FieldOptions in descriptor.proto](https://github.com/protocolbuffers/protobuf/blob/433ca3e/src/google/protobuf/descriptor.proto#L637-L802)) to store application-specific data like translations, relationships or categories. We compile our proto files to file descriptor sets and load those with grpc-loader. Unfortunately custom options on field level are currently not exposed by the objects created by proto-loader.

### Describe the solution you'd like
Similar to #2711 for custom options on method level we would like proto-loader to expose custom options on field level.

Would you accept a pull request for this?