psa: Add storage implementation for files

Add new functions, psa_load_persistent_key(),
psa_free_persistent_key_data(), and psa_save_persistent_key(), for
managing persistent keys. These functions load to or save from our
internal representation of key slots. Serialization is a concern of the
storage backend implementation and doesn't abstraction-leak into the
lifetime management code.

An initial implementation for files is provided. Additional storage
backends can implement this interface for other storage types.
diff --git a/library/version_features.c b/library/version_features.c
index ffad82f..7ef8997 100644
--- a/library/version_features.c
+++ b/library/version_features.c
@@ -687,6 +687,12 @@
 #if defined(MBEDTLS_PSA_CRYPTO_C)
     "MBEDTLS_PSA_CRYPTO_C",
 #endif /* MBEDTLS_PSA_CRYPTO_C */
+#if defined(MBEDTLS_PSA_CRYPTO_STORAGE_C)
+    "MBEDTLS_PSA_CRYPTO_STORAGE_C",
+#endif /* MBEDTLS_PSA_CRYPTO_STORAGE_C */
+#if defined(MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C)
+    "MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C",
+#endif /* MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C */
 #if defined(MBEDTLS_RIPEMD160_C)
     "MBEDTLS_RIPEMD160_C",
 #endif /* MBEDTLS_RIPEMD160_C */