[BACK] Add endpoints to upload and download user's financial transaction data as a CSV.

[ArielMAJ]

Type of feature

🍕 Feature

Current behavior

There's no way to download or upload user's data.

Suggested solution

• Add endpoint to download user's data as a CSV;
• Add endpoint to upload user's data as a CSV;

Additional context

Related frontend issue: ArielMAJ/finances-frontend#48