fix #11577: have checksum provider list available hashers

This should remove a client call to the server on import, while making the client robust against the server preferring a hasher that the client does not implement.

To test, make sure that import still works, still writing appropriate values for `hasher`, `hash` into the `originalfile` table; perhaps also experiment with setting `omero.checksum.supported` in `etc/omero.properties`.