The calls-gitaly tooling is not available when calling
`GraphqlHelpers#resolve` and `#resolve_field`, because it was
implemented as schema instrumentation which must be called from
`Schema.execute`. This changes to use field extensions that are:

- simpler (less code, easier to read)
- can be applied selectively (here we can avoid the runtime cost in
  prod)
- not deprecated; to use the intepreter, we will need to make this move

Changes:
 - Adds `#may_call_gitaly` method to BaseField