enb: Implement initial RF emulation API

Two implementations are provided:
* Amarisoft Ctrl interface (websocket)
* Mini-Circuits Programmable Attenuator (HW, HTTP API) [1]

in Amarisoft ENBs, if no rfemu is configured explicitly, the Ctrl
interface one is used by default, while still being possible to use the
HW one.

[1] https://www.minicircuits.com/pdfs/RC4DAT-6G-60.pdf

Change-Id: Ie98a3fb9bcd2b87b96ecbb5b79e0f53981892a32
diff --git a/src/osmo_gsm_tester/resource.py b/src/osmo_gsm_tester/resource.py
index efb7d5a..d4757f3 100644
--- a/src/osmo_gsm_tester/resource.py
+++ b/src/osmo_gsm_tester/resource.py
@@ -111,6 +111,9 @@
         'enb[].cell_list[].cell_id': schema.UINT,
         'enb[].cell_list[].scell_list[]': schema.UINT,
         'enb[].cell_list[].dl_earfcn': schema.UINT,
+        'enb[].cell_list[].dl_rfemu.type': schema.STR,
+        'enb[].cell_list[].dl_rfemu.addr': schema.IPV4,
+        'enb[].cell_list[].dl_rfemu.ports[]': schema.UINT,
         'arfcn[].arfcn': schema.INT,
         'arfcn[].band': schema.BAND,
         'modem[].type': schema.STR,