cat: add sms_pp_download_envelope()

Put a helper for wrapping an SMS-DELIVER TPDU in the ENVELOPE
of TS 102 223 section 7.5.1 the assembly next to SMSPPDownload,
where the IEs already live, so it can be reused by other tooling,
for example for triggering scp81 sessions.

Change-Id: Id23227eac53d697f4f13a84e087c32bf1d60f474

pySim/cat.py

@@ -763,6 +763,22 @@ class SMSPPDownload(BER_TLV_IE, tag=0xD1,
                     nested=[DeviceIdentities, Address, SMS_TPDU]):
     pass
 
+
+def sms_pp_download_envelope(tpdu, call_number: str = '0123456') -> SMSPPDownload:
+    """TS 31.111 Section 7.1.1.2 wrap of a SMS-DELIVER TPDU in the ENVELOPE (SMS-PP Download)
+     call_number :
+     SMSC address to report, defined in TS 31.111 7.1.1.2 as
+    "the RP_Originating_Address of the Service Centre (TS-Service-Centre-Address, 3GPP TS 24.011)"
+    its presence is Conditional, and the note there says the UICC should be fine
+    if its missing, so for remote management its presence should suffice (?).
+    """
+    return SMSPPDownload(children=[
+        DeviceIdentities(decoded={'source_dev_id': 'network', 'dest_dev_id': 'uicc'}),
+        Address(decoded={'ton_npi': {'ext': False, 'type_of_number': 'unknown',
+                                     'numbering_plan_id': 'unknown'},
+                         'call_number': call_number}),
+        SMS_TPDU(decoded={'tpdu': b2h(tpdu.to_bytes())})])
+
 # TS 101 220 Table 7.17 + 31.111 7.1.1.3
 class SMSCBDownload(BER_TLV_IE, tag=0xD2,
                     nested=[DeviceIdentities, CBSPage]):