Added SLSCH MAC sub-header, updated MAC sub-headers based on spec. 38321

See merge request aburger/openairinterface5g!269

Following changes are made:
1) Added instructions in rfsim README.md for running
rfsimulator with updated sl_preconfiguration.conf location

Following changes are made:
1) Added SLSCH MAC subheader
2) Validated MAC SDU subheader
3) Added SL_BSR subheader
4) Modified & validated MAC subheaders processing on tx and rx

Updates covered in this commit:
1) Computed feedback slot based on PSFCH time gap, PSFCH period and HARQ
	    feedback
2) Generated PSFCH pdu
3) Implemented PSFCH scheduling
4) Allocated PRBs based on slot and subchannel
5) Updated data structures accordingly
6) Modified configuration computation for PSFCH PRB set
7) Changed computation for number of PSFCH symbols
8) Moved sl_preconfiguration.conf under NR-SIDELINK/CONF

Removed send_psfch_with_pucch command line argument

See merge request aburger/openairinterface5g!267

PSFCH Tx implementation: PSFCH PDU generation, scheduling and PRBs allocation

See merge request aburger/openairinterface5g!268

Removed send_psfch_with_pucch command line argument

See merge request aburger/openairinterface5g!267

Merge branch 'episys/ea/psfch_configurations' of gitlab.int-episci.com:aburger/openairinterface5g into episys/ea/psfch_configurations

Added functionality to read PSFCH configurations from conf file

See merge request aburger/openairinterface5g!266

a) Added PSFCH configurations in conf file for TX resource pool;
b) In case of pucch based psfch transmission, ignored number of psfch
symbols for number of pssch symbols calculation