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_its.c b/library/psa_crypto_storage_its.c
index bb0d0cd..a60a8f3 100644
--- a/library/psa_crypto_storage_its.c
+++ b/library/psa_crypto_storage_its.c
@@ -36,12 +36,12 @@
 #include "mbedtls/platform.h"
 #endif
 
-static psa_storage_uid_t psa_its_identifier_of_slot( psa_key_id_t key )
+static psa_storage_uid_t psa_its_identifier_of_slot( psa_key_file_id_t key )
 {
     return( key );
 }
 
-psa_status_t psa_crypto_storage_load( const psa_key_id_t key, uint8_t *data,
+psa_status_t psa_crypto_storage_load( const psa_key_file_id_t key, uint8_t *data,
                                       size_t data_size )
 {
     psa_status_t status;
@@ -57,7 +57,7 @@
     return( status );
 }
 
-int psa_is_key_present_in_storage( const psa_key_id_t key )
+int psa_is_key_present_in_storage( const psa_key_file_id_t key )
 {
     psa_status_t ret;
     psa_storage_uid_t data_identifier = psa_its_identifier_of_slot( key );
@@ -70,7 +70,7 @@
     return( 1 );
 }
 
-psa_status_t psa_crypto_storage_store( const psa_key_id_t key,
+psa_status_t psa_crypto_storage_store( const psa_key_file_id_t key,
                                        const uint8_t *data,
                                        size_t data_length )
 {
@@ -105,7 +105,7 @@
     return( status );
 }
 
-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 )
 {
     psa_status_t ret;
     psa_storage_uid_t data_identifier = psa_its_identifier_of_slot( key );
@@ -125,7 +125,7 @@
     return( PSA_SUCCESS );
 }
 
-psa_status_t psa_crypto_storage_get_data_length( const psa_key_id_t key,
+psa_status_t psa_crypto_storage_get_data_length( const psa_key_file_id_t key,
                                                  size_t *data_length )
 {
     psa_status_t status;