Adopt dynamic seq_arr in PDU Session handling (RRC PDU Session)

The RRC PDU Session struct is converted from a fixed-size array
to a dynamically sized seq_arr. This improves flexibility and
simplifies session management by eliminating arbitrary size limits
and enabling dynamic addition/removal of PDU sessions.

Helper functions and loops have been adapted to use the seq_arr API.
This change is a step toward more robust and maintainable handling
of PDU sessions in the gNB RRC layer.

The "status" state machine remains untouched in this commit and
is refactored in a later commit.

Also:

* add macro to push to seq_arr and return the new element
* move initialization of xid (RRC PDU Session struct) to
  find_pduSession (temporary, will be finalized in a later
  commit) since with the new seq_arr design the initialization
  is no longer possible in the UE context generation (also
  does not belong to it, initialize when adding a new PDU Session)

CMakeLists.txt

@@ -1893,6 +1893,7 @@ target_link_libraries(nr-cuup PRIVATE
                       CONFIG_LIB ITTI SCTP_CLIENT
                       GTPV1U e1ap f1ap
                       time_management
+                      alg
                       z dl pthread shlib_loader ${T_LIB})
 target_link_libraries(nr-cuup PRIVATE asn1_lte_rrc_hdrs asn1_nr_rrc_hdrs)
 if(E2_AGENT)