Refactor GCS deploy helper workflow as a composite action

[falquaddoomi]

I originally implemented #30 (issue #5) using a "helper" workflow to deploy GCP functions, .github/workflows/helper-deploy-func.yaml, that gets invoked via a workflow trigger by the workflows for specific functions.

Some time after, I stumbled across custom actions, specifically Composite Actions, which are IMHO a more elegant solution to reusing a bunch of steps with parameters than a "helper" workflow.

I've started refactoring the workflow as a composite action, and I thought I'd create an issue first to track the work.

[falquaddoomi]

Actually, now I'm not so sure; it seems there are multiple downsides to using composite actions, including:

1. not being able to inherit secrets; they have to be passed as env vars
2. all inputs being of type string
3. not being able to choose a runner (the runner in a composite action is "composite"), which i assume also means no ability to parallelize in the workflow

I think I'm going to close this for now and perhaps reopen if we find a reason to switch from a reusable workflow to a composite action. The work so far was pushed to the branch issue-35-deploy-composite-action for future reference.