Add X2 peers support for symmetry with XN peers.

As with Radio Units and cells organize a registry of peers and start to handle
that registry generally everywhere. The registry is still populated with only
LTE peers on enb and only NR peers on gnb, but the only place that is aware of
that is registry loading routine.

Tests will be added later as full tests for generic MultiRU.

Rendered gnb.cfg stays the same, but there is a small change in rendered enb
but the config remains with the previous semantic as before hereby patch:

```
$ ./pythonwitheggs slapos-render-config.py && xdiff -w config/{old,out}
```

```diff
diff --git a/config/old/enb.cfg b/config/out/enb.cfg
index 5842dfc22..1a2c41b6b 100644
--- a/config/old/enb.cfg
+++ b/config/out/enb.cfg
@@ -22,6 +22,7 @@
     },
   ],
   gtp_addr: "127.0.1.1",
+  x2_peers: [],
   enb_id: 0x1A2D0,

   // LTE cells
```