chore: Migrate pubsub synth.py to bazel (#124)
chore: Migrate pubsub synth.py to bazel (#124)

This PR migrates only synth.py but does not commit the regenerated files. The generation was tested and it works, the updated files are not commited due to breaking changes not related to bazel migration.
googleapis/java-pubsub,googleapis/java-pubsub,googleapis/java-pubsub