[feature] add ranges to blobs backend

src/leap/soledad/server/_blobs/errors.py

src/leap/soledad/server/_blobs/fs_backend.py

src/leap/soledad/server/_blobs/resource.py

src/leap/soledad/server/interfaces.py

tests/blobs/test_fs_backend.py

tests/server/test_blobs_server.py

tests/server/test_incoming_server.py