Support encoding an owner in key file IDs

Differentiate between _key identifiers_, which are always `uint32_t`,
and _key file identifiers_, which are platform-dependent. Normally,
the two are the same.

In `psa/crypto_platform.h`, define `psa_app_key_id_t` (which is always
32 bits, the standard key identifier type) and
`psa_key_file_id_t` (which will be different in some service builds).
A subsequent commit will introduce a platform where the two are different.

It would make sense for the function declarations in `psa/crypto.h` to
use `psa_key_file_id_t`. However this file is currently part of the
PSA Crypto API specification, so it must stick to the standard type
`psa_key_id_t`. Hence, as long as the specification and Mbed Crypto
are not separate, use the implementation-specific file
`psa/crypto_platform.h` to define `psa_key_id_t` as `psa_key_file_id_t`.

In the library, systematically use `psa_key_file_id_t`.

    perl -i -pe 's/psa_key_id_t/psa_key_file_id_t/g' library/*.[hc]
diff --git a/library/psa_crypto_storage.h b/library/psa_crypto_storage.h
index 74f9e23..7e5aae9 100644
--- a/library/psa_crypto_storage.h
+++ b/library/psa_crypto_storage.h
@@ -86,7 +86,7 @@
  * \retval PSA_ERROR_STORAGE_FAILURE
  * \retval PSA_ERROR_ALREADY_EXISTS
  */
-psa_status_t psa_save_persistent_key( const psa_key_id_t key,
+psa_status_t psa_save_persistent_key( const psa_key_file_id_t key,
                                       const psa_key_type_t type,
                                       const psa_key_policy_t *policy,
                                       const uint8_t *data,
@@ -117,7 +117,7 @@
  * \retval PSA_ERROR_STORAGE_FAILURE
  * \retval PSA_ERROR_DOES_NOT_EXIST
  */
-psa_status_t psa_load_persistent_key( psa_key_id_t key,
+psa_status_t psa_load_persistent_key( psa_key_file_id_t key,
                                       psa_key_type_t *type,
                                       psa_key_policy_t *policy,
                                       uint8_t **data,
@@ -134,7 +134,7 @@
  *         or the key did not exist.
  * \retval PSA_ERROR_STORAGE_FAILURE
  */
-psa_status_t psa_destroy_persistent_key( const psa_key_id_t key );
+psa_status_t psa_destroy_persistent_key( const psa_key_file_id_t key );
 
 /**
  * \brief Free the temporary buffer allocated by psa_load_persistent_key().