✨ Store Encryption Package File in Mongo

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Suggestion

The encryption package is currently saved in the file system because Mongo doesn't support file sizes over 16 Meg. However, storing it in the database would be cleaner.

### Possible Implementation

Use GridFS: https://www.mongodb.com/docs/manual/core/gridfs/

### Anything else?

See [election_service.py](https://github.com/microsoft/electionguard-python/pull/718/files#diff-99d0dc5a77c718208e6a3f7f001e7680f76e14ef42b3a0d783bae29a22426b04)

先閱讀 election_service.py 和連結的 pull request，了解目前如何將 encryption package 寫入檔案系統以及從檔案系統讀取。檢視 MongoDB GridFS，然後將完成條件定義為透過 MongoDB 儲存和擷取該 package，而不依賴檔案系統儲存。

由索引模型根據 Issue 內容生成。