reverseproxy: Implement health_uri, replaces health_path, supports query

[francislavoie]

Closes #4049

reverse_proxy localhost:54321 {
	health_uri /health?ready=1
	health_status 2xx
}

Also fixes a bug with health_status Caddyfile parsing, it would always only take the first character of the status code even if it didn't end with "xx". I fixed the same bug in #4021 but it's relevant here too because it affects the adapt test I wrote (will rebase #4021 after and remove the redundant fix)

[francislavoie]

Flaky tests 😢

[mholt]

So here's a curveball, what if we introduce health_uri and deprecate health_path? The URI is just the path + optional query string in this case. That way the user doesn't have to separate them, seems kinda odd to require them separate...

[francislavoie]

Sure, that works 👍 I'll rework it

[francislavoie]

@mholt Done 😄

[mholt]

Thanks for working on this!

[mholt]

Thank you for helping with this! This is a nice improvement.