Bluetooth: controller: Implemented PDU release timeout

Implemented:
-- Combining segments from multiple SDUs in a single PDU for TX
-- Release of PDU based on remaining capacity below threshold
-- Release of any PDUs in production on receiving a timeout for the
   upcoming event
-- Release of PDUs in production if source if destroyed
-- Synchronization flag to exclude multiple contexts in ISO-AL
-- New Kconfig BT_CTLR_ISO_TX_SEG_PLAYLOAD_MIN to allow definition of
   minimum TX segment size

Signed-off-by: Nirosharn Amarasinghe <niag@demant.com>