reverseproxy: add webtransport Caddyfile subdirective

Adds a `webtransport` subdirective to the `transport http {}` block of
reverse_proxy that sets the new WebTransport bool on the transport.
Takes no arguments; exclusivity with versions 3 is enforced at
Provision time so parse order doesn't matter.

Example:
    reverse_proxy https://backend:9443 {
        transport http {
            versions 3
            webtransport
            tls_insecure_skip_verify
        }
    }

Includes a Caddyfile-to-JSON adapt test round-tripping the new
subdirective.

caddytest/integration/caddyfile_adapt/reverse_proxy_webtransport.caddyfiletest

@@ -0,0 +1,47 @@
+:8443
+
+reverse_proxy https://backend:9443 {
+	transport http {
+		versions 3
+		webtransport
+		tls_insecure_skip_verify
+	}
+}
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":8443"
+					],
+					"routes": [
+						{
+							"handle": [
+								{
+									"handler": "reverse_proxy",
+									"transport": {
+										"protocol": "http",
+										"tls": {
+											"insecure_skip_verify": true
+										},
+										"versions": [
+											"3"
+										],
+										"webtransport": true
+									},
+									"upstreams": [
+										{
+											"dial": "backend:9443"
+										}
+									]
+								}
+							]
+						}
+					]
+				}
+			}
+		}
+	}
+}

modules/caddyhttp/reverseproxy/caddyfile.go

@@ -1315,6 +1315,15 @@ func (h *HTTPTransport) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 				return d.ArgErr()
 			}
 
+		case "webtransport":
+			// Accepts no arguments: `webtransport` alone enables it.
+			// Exclusivity with `versions 3` is enforced at Provision
+			// time so parsing is order-independent.
+			if d.NextArg() {
+				return d.Errf("webtransport does not take arguments")
+			}
+			h.WebTransport = true
+
 		case "compression":
 			if d.NextArg() {
 				if d.Val() == "off" {