Convert UDCDirectoryProvider to standard pattern

[DiamondJoseph]

Fixes #1355

Link to dodal PR (if required): #505
(remember to update setup.cfg with the dodal commit tag if you need it for tests to pass!)

This ensures that the DirectoryProvider is a singleton that will reliably be shared between all devices if e.g. copying a detector from an existing beamline. It also exposes it to the pre-processor that is being used for configuring devices per-Run.

To test:

1. Check that PandA would write into correct directory for parameters if it wrote to a directory

[olliesilvester]

Looks good, although if you agree with my suggested dodal changes then this will need to be updated before merging