Add a missing signal detector helper job

[sgratzl]

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

Summary

add a script and github action helper workflow to detect discrepancies between covidcast_meta and covidcast/meta, i.e. missing signals in the google sheet. will generate a file like this:

missing_db_signals.csv

[krivard]

👍