OPENTHREAD_CONFIG_TREL_PEER_TABLE_SIZE macro

@def OPENTHREAD_CONFIG_TREL_PEER_TABLE_SIZE Specifies the capacity of TREL peer table. Only non-zero value will be directly used for setting the TREL peer table capacity. Zero value lets the size to be determined by the OT stack itself which is derived based on other configurations such as a child table size, neighbor table size, etc.