fix: allow requests to /healthz without authentication

[wendtek]

Problem

Basic Auth support was added in #1777, however this disabled the ability to use AWS ELB health checks against the /healthz endpoint. This purpose of this change is to allow the /healthz endpoint to work for systems that check automatically and do not support authentication or it is desired to configure without authentication, including AWS ELB health checks and Kubernetes probes.

Solution

This PR changes the behavior of the middleware when it checks if Basic Auth is enabled to allow unauthenticated requests to /healthz in the same way that requests to /events were excepted from the check.

[wendtek]

I also just learned of another undocumented endpoint, /status. Will add that as well

[chenrui333]

Make sense, thanks @wendtek!

[wendtek]

@chenrui333 do you mind reviewing again? I came across another endpoint I didn't see in documentation that others are using as a health check

[chenrui333]

Thanks @wendtek!