continuously update SIB19 information for rfsim NTN LEO scenarios

Contains mainly these changes:

• gNB: add function nr_update_sib19() allowing to update SIB19 information
• rfsimulator: update SIB19 contents every frame for LEO channel simulations
• NR UE: improve initial timing advance calculation from SIB19

Before, we only considered ta_Common_r17 and the SAT position from SIB19 to compute the initial timing advance value. This is good enough if the satellite does not move too fast (e.g. GEO satellite).

Now we also consider ta_CommonDrift_r17, ta_CommonDriftVariant_r17 and the SAT velocity together with the epoch time from SIB19. This improves the accuracy of the initial TA computation, esp. for LEO satellite scenarios.