Reduce SSL setup overhead in async Kubernetes triggers

[xvega]

I extracted the cncf.kubernetes changes from #71349 so they can be reviewed and merged independently.

• Reuse the API client, connection pool, and SSL context when the hook’s configuration is cached.
• Preserve per-call clients for detected exec-based authentication.
• Release cached clients during pod and job trigger cleanup, including early returns and errors.
• Cover client reuse, close/reopen behavior, and cleanup paths.

Direct hook users must call await hook.close() when finished. Cached TLS material is retained until the client is recreated.

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Was generative AI tooling used to co-author this PR?

• No

[Miretpl]

I think that I approved this modification in the past, but after taking a bit more deeper look, I think that we have to do some modifications here to not make optimalization more important than operation stability.