Add a CU-UP load tester and improve GTP performance

This adds a simple tester program for the CU-UP. It sets up a number of UEs/bearers in the CU-UP via the E1 interface, then streams data in downlink/uplink (as seen by the CU-UP). More information is available in tests/nr-cuup/README.md.

Includes some fixes to properly verify that SCTP is installed, and use "the right cmake way" to use it in cmake code (see commit for details).

TBD:

• add documentation
• Leave only "non-risky" GTP optimizations (e.g., there was a function for lockless sending, has been removed to avoid API abuse).
• remove F1-C port numbers from all configs, they are not used