Generate constants for full method names

[pkwarren]

Is your feature request related to a problem? Please describe.
It would be convenient in some cases to have access to the full method name for each method. Previous attempts to add this data required separate plugins: bufbuild/plugins#264

Describe the solution you'd like
Add the full method name as a constant in the generated .go code.

Describe alternatives you've considered
n/a

Additional context
n/a

[bufdev]

I'd like to make sure this is necessary before we potentially pollute the generated code.

[akshayjshah]

I'd like to make sure this is necessary before we potentially pollute the generated code.

Ack. This comes up pretty often (#326, #383, Slack, in person with https://github.com/cuebox) - the most common use case is exempting some RPCs from logging or auth middleware without hard-coding a string literal. The best available workaround is not very good, so IMO this is worthwhile.

[bufdev]

Ok