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.c b/library/psa_crypto_storage.c
index b4e4076..42bd938 100644
--- a/library/psa_crypto_storage.c
+++ b/library/psa_crypto_storage.c
@@ -148,7 +148,7 @@
     return( PSA_SUCCESS );
 }
 
-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,
@@ -186,7 +186,7 @@
     mbedtls_free( key_data );
 }
 
-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,