reverseproxy: Add fallback for some policies, instead of always random

[francislavoie]

Closes #5486

This adds a new fallback option for some load balancing policies: query, header and cookie. See https://caddyserver.com/docs/caddyfile/directives/reverse_proxy#load-balancing

Previously, the default was to always choose a random upstream when using those policies when there's no value available. Now, the fallback policy is configurable.

Example usecases:

• cookie + least_conn, where the first request (with no cookie) gets the upstream with the fewest current requests, instead of a random one
• header + first where if a header isn't provided, the first available upstream is chosen instead; so essentially the first policy allows for primary/secondary behaviour normally, but allows for an escape hatch for testing the secondary upstream by using a header

In the Caddyfile, it looks like this:

reverse_proxy :8001 :8002 :8003 {
	lb_policy cookie LB-Cookie-Name SuperSecretHmacKey {
		fallback least_conn
	}
}

Also, an insane case like this could be done:

reverse_proxy :8001 :8002 :8003 {
	lb_policy cookie LB-Cookie-Name SuperSecretHmacKey {
		fallback cookie A-Different-Cookie {
			fallback cookie Another-Different-Cookie {
				fallback cookie Yet-Another-Different-Cookie
			}
		}
	}
}

There's lots of boilerplate here unfortunately, I tried to reduce it where I could. Added a new test for cookie + first since it's an easy case. Now, the policies need provisioning, some default value cases were hoisted to the Provision function.

I used Copilot to speed up writing this PR, it wrote a lot of the boilerplate for me. I've reviewed and tested the code it generated.

[mholt]

Cool, I think this makes sense. Thank you!

PS. And thanks for the AI disclosure 💯