Implement PSA ITS over files

Implement the PSA ITS API over stdio files.
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index e9372be..244dc34 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -58,6 +58,7 @@
     psa_crypto_storage.c
     psa_crypto_storage_file.c
     psa_crypto_storage_its.c
+    psa_its_file.c
     ripemd160.c
     rsa.c
     rsa_internal.c