Draft: Resolve "writer state change events"

Closes #2479

Suggested by @andy.gotz. I added a wrapper to simplify usage:

from bliss.common.tango import DeviceProxy
from nexus_writer_service.utils.scan_utils import wait_writer_finished

proxy = DeviceProxy("idxy/bliss_nxwriter/mysession")
wait_writer_finished("1_loopscan", proxy=proxy)

If you have access to the Bliss session:

wait_writer_finished(scan.node.name, proxy=SCAN_SAVING.writer_object.proxy)

If you don't provided the proxy, it will use the HDF5 file.