feat: delete block endpoint

Blocks can now be deleted, if they have no references. A status
code of 204 is then returned.
A Constraint Error is raised by the metadata driver if there
are references, and 412 is returned as status code.

Endpoint:
    DELETE /v1.0/vaults/{vault}/blocks/{blockid}
This should do something if it fails. At the very least it should be in a try/except as, f.e, the Sqlite driver doesn't do that internally. While the Sqlite driver should probably do the try/except internally, it is still good to have some error handling here - at the very least to log it.