Add Metadata support to code generator

The code generator was not generating the necessary Get_FCPAffectedByMetadata_ function that is required for each metadata. This MR fixes that.

I also fixed the generation of the parameter and response doc comments (responses were documented with 'request.FieldName: ...' which was bothering me ;))

Further, in the servicer's __init__, I changed the calls to _inject_implementation to switch_to_<mode>_mode. That prevents code duplication and also simplifies things when passing additional arguments to the actual implementation classes since the adjustments have to be done only in one place now.