httpserver: Add experimental H2C support (#3289)

* reverse_proxy: Initial attempt at H2C transport/client support (#3218)

I have not tested this yet

* Experimentally enabling H2C server support (closes #3227)

See also #3218

I have not tested this

* reverseproxy: Clean up H2C transport a bit

* caddyhttp: Update godoc for h2c server; clarify experimental status

* caddyhttp: Fix trailers when recording responses (fixes #3236)

* caddyhttp: Tweak h2c config settings and docs

modules/caddyhttp/server.go

@@ -111,6 +111,17 @@ type Server struct {
 	// This field is not subject to compatibility promises.
 	ExperimentalHTTP3 bool `json:"experimental_http3,omitempty"`
 
+	// Enables H2C ("Cleartext HTTP/2" or "H2 over TCP") support,
+	// which will serve HTTP/2 over plaintext TCP connections if
+	// a client support it. Because this is not implemented by the
+	// Go standard library, using H2C is incompatible with most
+	// of the other options for this server. Do not enable this
+	// only to achieve maximum client compatibility. In practice,
+	// very few clients implement H2C, and even fewer require it.
+	// This setting applies only to unencrypted HTTP listeners.
+	// ⚠️ Experimental feature; subject to change or removal.
+	AllowH2C bool `json:"allow_h2c,omitempty"`
+
 	primaryHandlerChain Handler
 	errorHandlerChain   Handler
 	listenerWrappers    []caddy.ListenerWrapper