Add dissector for weird proprietary DLT_USER=162 format of 3GPP pcaps

We received some pcap files from a customer which contained protocol
traces in a proprietary format (generated by unknown software/tools).

The format uses DLT_USER=162 with some proprietary header that describes
the next-layer protocol (like SGsAP, RANAP, TCAP, MTP3) and various
other bits we have no idea about.

Let's try to figure out where the next-layer payload is and pass that
to existing wireshark dissectors.

dlt162_3gpp.lua

@@ -0,0 +1,67 @@
+-- wireshark LUA dissector for an unknown USER_DLT=162 protocol
+-- generated by some unknown equipment in a 3GPP cellular network.
+--
+-- (C) 2023 by Harald Welte <laforge@gnumonks.org>
+-- SPDX-License-Identifier: GPL-2.0+
+--
+-- Usage: Move (or symlink) this file to your "personal lua plugins"
+-- folder that can be found in the Wireshark Help->About
+-- Wireshark->Folders tab Windows: %APPDATA%\Wireshark\plugins.
+-- Unix-like systems: ~/.local/lib/wireshark/plugins.
+
+dlt162_protocol = Proto("dlt162", "DLT162 3GPP Protocol")
+
+local ranap_diss = Dissector.get("ranap")
+local mtp3_diss = Dissector.get("mtp3")
+local tcap_diss = Dissector.get("tcap")
+local sgsap_diss = Dissector.get("sgsap")
+
+local f_subprotocol = ProtoField.string("subprotocol", "Sub-Protocol")
+local f_subprotocol_len = ProtoField.uint8("subprotocol_len", "Sub-Protocol Length")
+
+dlt162_protocol.fields = {
+	f_subprotocol, f_subprotocol_len,
+}
+
+function dlt162_protocol.dissector(tvb, pinfo, tree)
+	pinfo.cols.protocol = dlt162_protocol.name
+
+	local subtree = tree:add(dlt162_protocol, tvb(), "DLT 162")
+
+	-- FIXME: verify that first 3 bytes are 00 01 00, as we don't know their meaning
+	local strlen = tvb(3,1):uint()
+	local str = tvb(4,strlen):string()
+	local len_offs
+	local diss
+	local payload_len
+
+	subtree:add(f_subprotocol, tvb(4, strlen))
+
+	if     str == "ranap" then
+		len_offs = 0x27
+		diss = ranap_diss
+	elseif str == "sgsap" then
+		len_offs = 0x2b
+		diss = sgsap_diss
+	elseif str == "tcap" then
+		len_offs = 0x23
+		diss = tcap_diss
+	elseif str == "mtp3" then
+		len_offs = 0x0b
+		diss = mtp3_diss
+	end
+
+	if len_offs then
+		subtree:add(f_subprotocol_len, tvb(len_offs, 1))
+		payload_len = tvb(len_offs, 1):uint()
+	end
+
+	if diss ~= nil and payload_len then
+		diss:call(tvb(len_offs+1, payload_len):tvb(), pinfo, tree)
+	end
+
+	return tvb:len()
+end
+
+function dlt162_protocol.init()
+end