caddyhttp: Implement http.request.uuid placeholder (#4285)

modules/caddyhttp/app.go

@@ -50,6 +50,7 @@ func init() {
 // `{http.request.body}` | The request body (⚠️ inefficient; use only for debugging)
 // `{http.request.cookie.*}` | HTTP request cookie
 // `{http.request.duration}` | Time up to now spent handling the request (after decoding headers from client)
+// `{http.request.uuid}` | The request unique identifier
 // `{http.request.header.*}` | Specific request header field
 // `{http.request.host.labels.*}` | Request host labels (0-based from right); e.g. for foo.example.com: 0=com, 1=example, 2=foo
 // `{http.request.host}` | The host part of the request's Host header