[WIP] slackevents: alert on merged PRs with cncf-cla: no label

[AaruniAggarwal]

Fixes #279
This PR adds Slack notifications for merged pull requests carrying the cncf-cla: no label.

The implementation introduces CLAAlert configuration, handles pull request merge events in the slackevents plugin, supports both repository and organization-level configuration, and includes unit tests.

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: AaruniAggarwal
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[kubernetes-prow]

Hi @AaruniAggarwal. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

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