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)

blockserver の既存の設定と synoptic の処理を調査し、文脈を把握するために後続チケット #7393 を確認してください。コマンドラインを使って、XML ファイルがどのようにアップロード、一覧表示、再読み込みされるかを追跡し、必要な解凍や dehexing も含めて確認してください。適切な XML テンプレートと場所で 3 つすべての受け入れ基準が機能すれば完了です。

索引モデルが issue の本文から書いたものです。