fssrv: add ExternalKeyManager (rights-id crypto) for hac2l

libraries/libstratosphere/include/stratosphere/spl/spl_api.hpp

@@ -105,4 +105,6 @@ namespace ams::spl {
 
     Result LoadPreparedAesKey(s32 slot, const AccessKey &access_key);
 
+    Result PrepareCommonEsTitleKey(AccessKey *out, const void *key_source, const size_t key_source_size, int generation);
+
 }