file_sys/registered_cache: Use regular const references instead of...

file_sys/registered_cache: Use regular const references instead of std::shared_ptr for InstallEntry()

These parameters don't need to utilize a shared lifecycle directly in
the interface. Instead, the caller should provide a regular reference
for the function to use. This also allows the type system to flag
attempts to pass nullptr and makes it more generic, since it can now be
used in contexts where a shared_ptr isn't being used (in other words, we
don't constrain the usage of the interface to a particular mode of
memory management).