plex: provide its API to the mesh as plex-api

kometa, tautulli and ombi reached plex by a hand-typed address - a public
name, a HAL network gateway - which the mesh cannot keep true. Plex now
provides plex-api at mesh scope and serves the server's port and scheme, so
the mesh tells each consumer where it is.

The port is written out rather than inferred: plex listens on five ports
(the server and four discovery ones), and the mesh only infers a provision's
port when a module listens on exactly one.

No grants and no provisioner: the credential is the server owner's
X-Plex-Token, which plex.tv issues and the mesh cannot mint. The operator
accepts it as the pair credential for each consumer (ADR 0092); each
consumer's step checks it against the server and writes nothing it refuses.

modules/plex/module.json

@@ -1,6 +1,18 @@
 {
   "module": "plex",
   "version": "1",
+  "provides": [
+    {
+      "name": "plex-api",
+      "scope": "mesh"
+    }
+  ],
+  "serves": {
+    "plex-api": {
+      "scheme": "http",
+      "port": 32400
+    }
+  },
   "capabilities": [
     "container-runtime"
   ],