feat: graceful shutdown to prevent 502 errors when oauth2-proxy is load balanced

[adelsz]

This PR introduces a new shutdownDuration configuration variable which allows the proxy to shutdown gracefully.

After receiving a termination signal, proxy will keep serving traffic for shutdownDuration but will start returning a 503 error for its readiness /ready endpoint signaling to the load balancer to not send new traffic to it.

After shutdownDuration, proxy shuts down as usual.

Motivation and Context

Fixes https://github.com/oauth2-proxy/oauth2-proxy/issues/3066

How Has This Been Tested?

1. Launch proxy
2. Send SIGTERM
3. Observe that proxy keeps serving traffic for shutdownDuration seconds
4. While timeout hasn't elapsed /ready endpoint returns an error
5. After timeout the proxy exits normally

Checklist:

• [x] My change requires a change to the documentation or CHANGELOG.
• [x] I have updated the documentation/CHANGELOG accordingly.
• [x] I have created a feature (non-master) branch for my PR.
• [ ] I have written tests for my code changes.

[adelsz]

Thanks for the review @tuunit and @dolmen. I have addressed your comments in the last commit.