Simple URR support

[pirog-spb]

I, as a telco engineer, expect to have usage reports at least in the Session Deletion Response in order to follow 3GPP specifications and to provide usage info to SMF/SPGW-C for subscriber session charging purpose.

According to TS 129 244: 6.3.4 PFCP Session Deletion Procedure

6.3.4.3 UP Function Behaviour
When the UP function receives a PFCP Session Deletion Request it shall:
...
- send the PFCP Session Deletion Response message with an acceptance cause if the PFCP session and associated rules are deleted successfully, and **include any pending Usage Report(s) in the message**.

Acceptance criteria:

• UPF sends Usage Report if it was previously requested by URR in Session Establishment/Modification
• Exact value of usage is 0, because lack of usage monitoring support on the ebpf side