psa_get_and_lock_key_slot_with_policy() function

Get the description of a key given its identifier and policy constraints and lock it. The key must have allow all the usage flags set in \p usage. If \p alg is nonzero, the key must allow operations with this algorithm. If \p alg is zero, the algorithm is not checked. In case of a persistent key, the function loads the description of the key into a key slot if not already done. On success, the returned key slot has been registered for reading. It is the responsibility of the caller to then unregister once they have finished reading the contents of the slot. The caller unregisters by calling psa_unregister_read() or psa_unregister_read_under_mutex(). psa_unregister_read() must be called if and only if the caller already holds the global key slot mutex (when mutexes are enabled). psa_unregister_read_under_mutex() encapsulates the unregister with mutex lock and unlock operations.