reverseproxy: Add handle_response blocks to reverse_proxy (#3710) (#4021)

* reverseproxy: Add `handle_response` blocks to `reverse_proxy` (#3710)

* reverseproxy: complete handle_response test

* reverseproxy: Change handle_response matchers to use named matchers

reverseproxy: Add support for changing status code

* fastcgi: Remove obsolete TODO

We already have d.Err("transport already specified") in the reverse_proxy parsing code which covers this case

* reverseproxy: Fix support for "4xx" type status codes

* Apply suggestions from code review

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

* caddyhttp: Reorganize response matchers

* reverseproxy: Reintroduce caddyfile.Unmarshaler

* reverseproxy: Add comment mentioning Finalize should be called

Co-authored-by: Maxime Soulé <btik-git@scoubidou.com>
Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -31,6 +31,7 @@ import (
 	"time"
 
 	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
 	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
 	"github.com/caddyserver/caddy/v2/modules/caddyhttp/headers"
 	"go.uber.org/zap"
@@ -127,6 +128,12 @@ type Handler struct {
 	Transport http.RoundTripper `json:"-"`
 	CB        CircuitBreaker    `json:"-"`
 
+	// Holds the named response matchers from the Caddyfile while adapting
+	responseMatchers map[string]caddyhttp.ResponseMatcher
+
+	// Holds the handle_response Caddyfile tokens while adapting
+	handleResponseSegments []*caddyfile.Dispenser
+
 	ctx    caddy.Context
 	logger *zap.Logger
 }