Add nightly release workflow for release candidates

[desertaxle]

This PR adds a workflow to release a new 3.0 release candidate every night a 12am PST. The workflow will only create a release if there are changes to .py files or the Python requirements. The workflow will generate release notes and create a GitHub release (marked as a pre-release), triggering the existing publish workflows. These workflows will need to be manually approved for now. Once we have confidence in this workflow, we can deploy to a different GH environment to remove the need for manual approval.

Example

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[aaazzam]

:hellyeah:

[cicdw]

LGTM - I don't see where the prod environment is set that would require approval though?

[desertaxle]

LGTM - I don't see where the prod environment is set that would require approval though?

This workflow won't directly require approval. It will trigger the 'Publish Python package' workflow by creating a release in GitHub, and that workflow will require approval.

I think we'll need to set up a new environment for pre-releases, but we'll cross that bridge when we get there.