RRC multi-cell: add cell management library with lookup and cleanup

This commit introduces a new library module (rrc_cell_management.c/h)
to centralise functions that provides cell lookup, DU-specific cell
management, and cell lifecycle managements.

The design uses a dual-tree architecture:
- Global cell tree: RB-tree at RRC level (rrc->cells) keyed by cell_id,
  used for O(log N_CELL) lookups and for maintaining num_cells.
- DU-specific storage: seq_arr_t per DU (du->cells) for iteration and
  management of cells belonging to each DU.

API added:
- Global tree: get_cell_by_cell_id()(finds cell using global RB-tree
  lookup (O(log N_CELL) complexity); rrc_add_cell(); rrc_rm_cell()
  (rrc_rm_cell removes from tree, decrements num_cells, and frees the
  cell via rrc_free_cell_container).
- DU-local seq_arr: rrc_get_cell_for_du(), rrc_get_cell_by_pci_for_du(),
  rrc_add_cell_to_du() (with duplicate cell_id/PCI checks).
- Cleanup: rrc_free_cell_container() (frees ASN.1 MIB/SIB1/MTC and
  container

Build: add rrc_cell_management static library in openair2/RRC/NR;
link it into nr_rrc from top-level CMakeLists.txt.

CMakeLists.txt

@@ -342,7 +342,7 @@ target_link_libraries(lte_rrc PRIVATE nr_rrc)
 #NR RRC
 #######
 add_library(nr_rrc ${OPENAIR2_DIR}/RRC/NR/MESSAGES/asn1_msg.c)
-target_link_libraries(nr_rrc PUBLIC asn1_nr_rrc asn1_lte_rrc)
+target_link_libraries(nr_rrc PUBLIC asn1_nr_rrc asn1_lte_rrc rrc_cell_management)
 
 # S1AP
 ##############