Generalize Bigtable's regenerate scripts to applicable to other clients

[igorbernstein2]

The new Bigtable client has 2 scripts to help test GAPIC config changes. The scripts allow the developer to make a change in a local googleapis checkout and see how the changes will look in api-client-staging and google-cloud-java. It would be nice to open these scripts to other clients.

Also, the update script is currently pretty careless about overwriting local changes, this will need to narrowed down to autogenerated files to avoid destroying local changes

[igorbernstein2]

This came out of discussion in #2841. I was planning on generalizing the scripts once the bigtable client stabilizes. Should I not be planning to do this?

[yihanzhen]

Sorry I thought this issue was used to track the merged PR.

[chingor13]

There is now the utilities/batch_generate_apis.py script and yoshi-team is using synthtool to regenerate clients in #3765.