feat: add external property to coder_app (#5425)

* Add schema

* feat: add `external` property to `coder_app`

This allows exposing applications that open an external URL.

coderd/database/dump.sql

@@ -439,7 +439,8 @@ CREATE TABLE workspace_apps (
     health workspace_app_health DEFAULT 'disabled'::workspace_app_health NOT NULL,
     subdomain boolean DEFAULT false NOT NULL,
     sharing_level app_sharing_level DEFAULT 'owner'::app_sharing_level NOT NULL,
-    slug text NOT NULL
+    slug text NOT NULL,
+    external boolean DEFAULT false NOT NULL
 );
 
 CREATE TABLE workspace_builds (