gapic_metadata.proto should be updated automatically

[blakeli0]

gapic_metadata.proto currently downloads from a fixed commitish of googleapis.

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 to java-common-protos.