Draft: Add nr_dlbench: standalone DL scheduler + PHY TX benchmark

Standalone executable that measures wall-clock time of the full DL pipeline as a function of the number of connected UEs. Uses the real proportional-fair scheduler (gNB_dlsch_ulsch_scheduler) and real PHY TX chain (phy_procedures_gNB_TX). No radio or real UEs needed.

Compiles with ninja nr_dlbench from ran_build/build

after increasing the max number of UEs in common/openairinterface5g_limits.h to say 128 you can run with e.g.: ./nr_dlbench -u 128 -n 10000 -R 273 -m 1 -P -S 1 -B 0.2