allow generating methods for each field

[egonelbre]

Protobuf generates method to allow accessing each field. This also means that when some field is renamed or changed you can add a custom method that handles the backwards compatibility.

However, it does come with a significant cost, because the messages end up in interfaces -- which in turn means none of the methods can properly elided -- most of the time.

There should be an option to generate them, in case it's needed. This will also make transitioning storj.io/storj easier.

[storj-gerrit]

Change protoc-gen-pico: generate enum accessors mentions this issue.