chore: reduce log volume on server startup (#16608)

Addresses https://github.com/coder/coder/issues/16231.

This PR reduces the volume of logs we print after server startup in
order to surface the web UI URL better.

Here are the logs after the changes a couple of seconds after starting
the server:

<img width="868" alt="Screenshot 2025-02-18 at 16 31 32"
src="https://github.com/user-attachments/assets/786dc4b8-7383-48c8-a5c3-a997c01ca915"
/>

The warning is due to running a development site-less build. It wouldn't
show in a release build.

provisionersdk/serve.go

@@ -25,9 +25,10 @@ type ServeOptions struct {
 	// Listener serves multiple connections. Cannot be combined with Conn.
 	Listener net.Listener
 	// Conn is a single connection to serve. Cannot be combined with Listener.
-	Conn          drpc.Transport
-	Logger        slog.Logger
-	WorkDirectory string
+	Conn                drpc.Transport
+	Logger              slog.Logger
+	WorkDirectory       string
+	ExternalProvisioner bool
 }
 
 type Server interface {