mgcp: Be able to parse a Osmocom transaction ID

the namespace nat-NUMBER is not valid according to the grammar.
Add an optional extension for it and add a testcase. In case
we get too many extensions we might introduce a MGCPGrammar
subclass for osmocom.

callagent/Tests.st

@@ -361,6 +361,13 @@ PP.PPCompositeParserTest subclass: MGCPParserTest [
                 nextPutAll: 'M: recvonly'; cr; nl]
     ]
 
+    MGCPParserTest class >> natDLCXMessage [
+        ^String streamContents: [:stream |
+            stream
+                nextPutAll: 'DLCX nat-0 1@mgw MGCP 1.0'; cr; nl]
+
+    ]
+
     parserClass [
         ^MGCPParser
     ]
@@ -374,6 +381,14 @@ PP.PPCompositeParserTest subclass: MGCPParserTest [
         self assert: crcx asDatagram equals: self class crcxMessage.
     ]
 
+    testParseDLCX [
+        | dlcx |
+        dlcx := self parse: self class natDLCXMessage.
+
+        self assert: dlcx class verb equals: 'DLCX'.
+        self assert: dlcx asDatagram equals: self class natDLCXMessage.
+    ]
+
     testRespParse [
         | nl res sdp |
         nl := Character cr asString, Character nl asString.

grammar/MGCPGrammar.st

@@ -71,7 +71,9 @@ PP.PPCompositeParser subclass: MGCPGrammar [
 
     transaction_id [
         <category: 'grammar-cmd'>
-        ^ ((#digit asParser) min: 1 max: 9) flatten
+        "Add Osmocom extension that starts with 'nat-'"
+        ^ ((#digit asParser) min: 1 max: 9) flatten /
+          ('nat-' asParser, (#digit asParser) min: 1 max: 9) flatten
     ]
 
     endpointName [