reverseproxy: Dynamic upstreams (with SRV and A/AAAA support) (#4470)

* reverseproxy: Begin refactor to enable dynamic upstreams

Streamed here: https://www.youtube.com/watch?v=hj7yzXb11jU

* Implement SRV and A/AAA upstream sources

Also get upstreams at every retry loop iteration instead of just once
before the loop. See #4442.

* Minor tweaks from review

* Limit size of upstreams caches

* Add doc notes deprecating LookupSRV

* Provision dynamic upstreams

Still WIP, preparing to preserve health checker functionality

* Rejigger health checks

Move active health check results into handler-specific Upstreams.

Improve documentation regarding health checks and upstreams.

* Deprecation notice

* Add Caddyfile support, use `caddy.Duration`

* Interface guards

* Implement custom resolvers, add resolvers to http transport Caddyfile

* SRV: fix Caddyfile `name` inline arg, remove proto condition

* Use pointer receiver

* Add debug logs

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

modules/caddyhttp/reverseproxy/admin.go

@@ -87,7 +87,7 @@ func (adminUpstreams) handleUpstreams(w http.ResponseWriter, r *http.Request) er
 			return false
 		}
 
-		upstream, ok := val.(*upstreamHost)
+		upstream, ok := val.(*Host)
 		if !ok {
 			rangeErr = caddy.APIError{
 				HTTPStatus: http.StatusInternalServerError,
@@ -98,7 +98,6 @@ func (adminUpstreams) handleUpstreams(w http.ResponseWriter, r *http.Request) er
 
 		results = append(results, upstreamStatus{
 			Address:     address,
-			Healthy:     !upstream.Unhealthy(),
 			NumRequests: upstream.NumRequests(),
 			Fails:       upstream.Fails(),
 		})