Replace SendGrid with Gmail OAuth for release announcements

[saschagrunert]

What type of PR is this?

/kind feature
/kind cleanup

What this PR does / why we need it:

Replaces the SendGrid email integration with Gmail API using Google OAuth
for sending release announcements via krel announce send. SendGrid has
become unreliable (emails not delivered, accounts deactivated after
inactivity). The Gmail OAuth flow requires no setup — a browser opens for
authorization on each run using embedded OAuth credentials from the
k8s-release GCP project.

Which issue(s) this PR fixes:

Fixes #3360

Special notes for your reviewer:

• The embedded OAuth client ID and secret are not secrets per
  Google's documentation
  for "Desktop app" type OAuth clients. They only identify the application.
  User security is provided by the interactive consent flow.
• The OAuth app is managed in the
  k8s-release
  Google Cloud project.
• --no-browser flag supports headless environments (prints URL, user
  pastes redirect URL back).
• An EmailSender interface is introduced so SMTP support can be added
  later.

Does this PR introduce a user-facing change?

Replace SendGrid with Gmail OAuth for `krel announce send`. No setup
required — a browser opens for Google authorization on each run.
Use `--no-browser` for headless environments.

[saschagrunert]

/hold
/retest

for testing and approval.

PTAL @kubernetes/release-engineering

[saschagrunert]

/retest

[saschagrunert]

/unhold

It works: https://groups.google.com/g/kubernetes-announce-test/c/ewnLd_BMY0s 🥳

I published the app in the Google Cloud console for k8s-release, so no further action should be required from our side if no automation disables the API: https://console.cloud.google.com/auth/audience?project=k8s-release

[cpanato]

thanks!

[xmudrii]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, saschagrunert, xmudrii

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