Support http auth for health checks

[HLFH]

It would be great to support two parameters:

• health-check-username
• health-check-password

Example of Docker app that needs HTTP auth: Transmission.
https://hub.docker.com/r/linuxserver/transmission

kamal-proxy deploy does not work when the root URL requires HTTP auth.

[zdehasek]

Yes, this feature would be really cool.
I’m currently deploing Prometheus using HTTP authentication, and it would be great if Kamal Proxy could handle this. Otherwise, I won’t be able to deploy it at all.

We don’t even need new parameters: HTTP auth could be set like this: user:password@example.org. So, if the proxy allows providing a full URL instead of just the path, that would be perfect.

I am not a go developer, but if PR like this will be merged, I am happy to do my best :)