feat: support requesting numeric enum rest encoding

[noahdietz]

Support the top-level REST descriptor config option numericEnums which enables inclusion of the $alt=json;enum-encoding=int system parameter that requests that enums be encoded as integers instead of strings. This is a Google System parameter. This is an API-wide configuration hence its spot at the same level as interfaces

This will be coupled with a generator change to include the REST descriptor config option in the generated output when a generator option is supplied.

[noahdietz]

@bshaffer this is ready for review now.

[vchudnov-g]

LGTM, but I'll PHP experts give the final approval

[bshaffer]

Approved but one thing that surprises me about this is nothing is required client side to prepare for the response? Is that because we expect it to be handled automatically by protobuf? And would we benefit from any kind of mocking / test case for this scenario?

[noahdietz]

Is that because we expect it to be handled automatically by protobuf?

Yes, protobuf (de)serialization handles numbers or strings for enum fields. I tested it manually before, during the exploration phase of the encoding stuff. Most protobuf JSON serializers handle either string or number for enum fields smoothly.