Create numtracker-friendly internal parameter models

[olliesilvester]

When using Bluesky with BlueAPI + numtracker, there are a few parameters which are baked into our common components which we will now be getting from other places:

• visit: BlueAPI will add this to the run engine metadata as "instrument_session", so we can access this within callbacks rather than as a separate parameter
• storage_directory: StandardDetector is given a PathProvider which uses numtracker, so this directory is calculated for us

[olliesilvester]

We currently calculate our snapshot directory using the storage directory, so this will need changing too.