Store config in a database

Where possible, we should move away from storing config in files, and instead store config in a database, such as redis. This would reduce our reliance on the file system and allow for further standardisation with regards to file formats and filepaths (instead of a filepath we'd just need a key).

Ideally this could be done in a way where we could gradually transition from files to database, so both options should work for the time being. A good first candidate to test on would be `XpdfCrystalLookupTable`, config file at `dls_sw/i15-1/software/daq_configuration/xpdf_crystal_lut.txt`, as this file is never read by GDA.

## Acceptance Criteria
- Config can be stored in a database and read by the server from there
- Backwards compatibility with config files is maintained