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)

Inspecciona la configuración y el manejo de synoptic existentes de blockserver, y lee el ticket posterior #7393 para obtener contexto. Usa la línea de comandos para rastrear cómo se cargan, enumeran y vuelven a leer los archivos XML, incluido cualquier descomprimido o dehexing necesario; se considera terminado cuando los tres criterios de aceptación funcionan con la plantilla XML y la ubicación adecuadas.

Escrito por el modelo de indexación a partir del texto del issue.