When using Numtracker, allow writing to arbitrary subfolder within instrument_session folder

We are currently unable to write to subfolder within the instrument session folder.

The start document path provider always returns the folder. We need to be able to define any subfolder within the directory and without some convoluted way of doing it.

Eg you should be able to pass the instrument_session/ARBITRARILY_PATH and numtracker and blueapi should be able to consume this properly.

This is especially important when it comes to file space. In that often scientist's will want to write to the /tmp directory within the instrument_sessiom folder.

But also from a scientific point of view because it often makes sense to organise data sets together. Ie if they are all from the same sample