feat(gengapic): add snippets

[quartzmo]

The snippet output added in this change is known to differ from legacy snippetgen output as follows:

1. In new snippet metadata, resultType pointer values contain the * prefix. (In current metadata, the * does not appear.)
2. In new snippet file license headers, https replaces current headers' usage of http.
3. In new snippet file "generated by" header comments, the source is changed to the protoc plugin.
4. In new snippet file imports, a blank line that currently appears before iam imports is now removed. Note that this decreases the segments.end value by 1 in the new snippet metadata.

           datacatalog "cloud.google.com/go/datacatalog/apiv1beta1"
   -
           iampb "google.golang.org/genproto/googleapis/iam/v1"
5. In new snippet metadata, clientMethod.method values are changed to the actual protobuf source values for mixin methods:

            "method": {
              "shortName": "GetLocation",
   -          "fullName": "google.cloud.kms.v1.EkmService.GetLocation",
   +          "fullName": "google.cloud.location.Locations.GetLocation",
              "service": {
   -            "shortName": "EkmService",
   -            "fullName": "google.cloud.kms.v1.EkmService"
   +            "shortName": "Locations",
   +            "fullName": "google.cloud.location.Locations"
              }

[snippet-bot]

Here is the summary of possible violations 😱

There are 2 possible violations for not having product prefix.

• internal/gengapic/snippets.go:107, tag %s
• internal/gengapic/snippets_test.go:205, tag _pkg_generated_Foo_%s_sync

There are format violations for 2 region tags.

• internal/gengapic/snippets.go:107, tag %s
• internal/gengapic/snippets_test.go:205, tag _pkg_generated_Foo_%s_sync

The end of the violation section. All the stuff below is FYI purposes only.

---

Here is the summary of changes.

You are about to add 3 region tags.

• internal/gengapic/snippets.go:107, tag %s
• internal/gengapic/snippets_test.go:205, tag _pkg_generated_Foo_%s_sync
• internal/gengapic/testdata/snippet.want:17, tag bigquerymigration_v2_generated_MigrationService_CreateMigrationWorkflow_sync

---

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[noahdietz]

This is pretty cool, @quartzmo well done.

[noahdietz]

Can you add a new config file for SnippetBot to stop it from bothering us about region tags in generator code? Add the following to .github/snippet-bot.yml:

# The generator prints region tags and snippet-bot wants to correct
# the printing code.
ignoreFiles:
  - "internal/**"

[codyoss]

Just a couple of small things

[noahdietz]

I am confused why changes that have already landed in main are showing up in the diffs here, is anyone else seeing that? Like ci.yaml and deps.yaml changing to bazel verison 6.0.0. And the CHANGELOG.md including the latest release...why are they in this PR?

[quartzmo]

@noahdietz I also see those diffs in the file view for this PR in GH. But I do not see them locally. The following returns no differences:

git diff main -- .github/workflows/ci.yaml

I guess maybe GH might be confused by so many commits?

[noahdietz]

Good call on doing the conditional refactor now, it made things a lot cleaner.