feat: add a new feature enablement mechanism

[shollyman]

This PR adds a new feature enablement mechanism to the Go GAPIC
generator. The intent is to pass behavior enablement via protoc flags,
with support for the old explicit allowlists until we can transition
feature controls to downstream invocation callsites (e.g. bazel rules, librariangen).

This PR also migrates three existing internal features to use the new
feature mechanism: MTLS support, wrapper type support in list RPCs,
and ordered header generation.

[quartzmo]

This is a great improvement. I love the more concise conditionals in the production generation code:

if g.cfg.FeatureEnabled(...) {

[shollyman]

This is a great improvement. I love the more concise conditionals in the production generation code:

if g.cfg.FeatureEnabled(...) {

The level at which the check is expressed is something I've been struggling with as well. Should we hoist the check function up to the generator or leave it on the config? e.g. if g.FeatureEnabled(...) { instead of dotting into the cfg field.

[shollyman]

The level at which the check is expressed is something I've been struggling with as well. Should we hoist the check function up to the generator or leave it on the config? e.g. if g.FeatureEnabled(...) { instead of dotting into the cfg field.

I went ahead and moved the check function up to the generator, so its even simpler.

if g.featureEnabled(FooFeature) { // do the thing }

[hongalex]

LGTM but want approval from @quartzmo so won't include the approval