caddyhttp: New experimental handler for intercepting responses (#6232)

* feat: add generic response interceptors

* fix: cs

* rename intercept

* add some docs

* @francislavoie review (first round)

* Update modules/caddyhttp/intercept/intercept.go

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

* shorthands: ir to resp

* mark exported symbols as experimental

---------

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

caddytest/integration/intercept_test.go

@@ -0,0 +1,34 @@
+package integration
+
+import (
+	"testing"
+
+	"github.com/caddyserver/caddy/v2/caddytest"
+)
+
+func TestIntercept(t *testing.T) {
+	tester := caddytest.NewTester(t)
+	tester.InitServer(`{
+			skip_install_trust
+			admin localhost:2999
+			http_port     9080
+			https_port    9443
+			grace_period  1ns
+		}
+	
+		localhost:9080 {
+			respond /intercept "I'm a teapot" 408
+			respond /no-intercept "I'm not a teapot"
+
+			intercept {
+				@teapot status 408
+				handle_response @teapot {
+					respond /intercept "I'm a combined coffee/tea pot that is temporarily out of coffee" 503
+				}
+			}	
+		}
+		`, "caddyfile")
+
+	tester.AssertGetResponse("http://localhost:9080/intercept", 503, "I'm a combined coffee/tea pot that is temporarily out of coffee")
+	tester.AssertGetResponse("http://localhost:9080/no-intercept", 200, "I'm not a teapot")
+}