Save and Restore Device Settings - Blockserver

As someone implementing a method to save and restore PV values I want the blockserver to handle the files and their writing as it does in other areas.

This is a follow on ticket from #7393

## Acceptance Criteria
- [ ] The blockserver can receive an XML file with the appropriate template and save it to the appropriate location
- [ ] The blockserver can display a list of available files
- [ ] The blockserver can read and send an appropriate XML file

## Notes
- The blockserver already does similar things for items such as configs and synoptics
- This is about making the data available, and this should be possible on the command line (although you may need to do some unzipping and dehexing)