Add the ability in krel announce to send emails via Google OAuth for release managers using personal gmail accounts.

[jimangel]

What would you like to be added:

I would like to add Google OAuth support to krel announce with a limited scope that allowed krel to "send emails on your behalf." The code would issue a token valid for 1h with the intention that is refreshed upon each execution.

I created a demo here: https://github.com/jimangel/oauth-gmail-test

Slack convo about this demo: https://kubernetes.slack.com/archives/CJH2GBF7Y/p1699902725642849

It seems like it would work, I would just need some help figuring out the best way to add it into the krel codebase.

Why is this needed:

Currently krel announce has options to send post-release announce emails via SendGrid's API (or HTML output for copy / pasting). SendGrid, to combat spam (and possibly other reasons), has become increasingly unreliable:

@xmudrii has opened #3175 to explore alternatives and I'm reluctant to combine this issue with it as it only solves for senders that use gmail. There might be a more robust, bulk-sender-friendly, option that can be used by the entire release management team.

/sig release
/area release-eng
/kind feature
/priority important-longterm

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/lifecycle stale

[xmudrii]

/remove-lifecycle stale

[jimangel]

Checking in here too @xmudrii, happy to see this through to completion in Mid-June.