gapic_metadata.proto should be updated automatically

gapic_metadata.proto currently downloads from a fixed [commitish of googleapis](https://github.com/googleapis/google-cloud-java/blob/8a9fab9ed555bf3fb554924071ca361f5f988270/sdk-platform-java/gapic-generator-java/pom.xml#L137).

There are two main issues with this approach
- This caused toil that we have to manually update the commitish whenever we need to use new features from the proto.
- Always downloading from Github makes the build non-hermetic and could cause flaky build if Github is not responsive.

Possible solution: We may want to use automations to sync it from [googleapis](https://github.com/googleapis/googleapis/blob/master/gapic/metadata/gapic_metadata.proto) to [java-common-protos](https://github.com/googleapis/google-cloud-java/tree/main/java-common-protos).