Update implementation to use non-deprecated api calls

[iwittkau]

trafficstars

I updated the implementation to use the newer grpc/encoding package.

• update codec
• move codec to sub-package
• add tests
• fix existing tests
• update docs

To use grpc/encoding Codec interface the proxy has to register the codec as proto. Otherwise the marshalling/unmarshalling will fail. The proxy codec has to completely overwrite the proto codec behavior. I extended the codec test to make sure regular proto message get marshaled/unmarshalled correctly.

[asim]

Will this be merged anytime soon?

[Virtual-felix]

I'm also waiting for this to be merged

[jose-airspace]

Will it ever be merged? Is this repo to maintained anymore @mwitkow ? If you no longer want to maintain can you transfer to someone that does or give someone rights to merge PRs?