Add enpoints to generate bank transfer xml files

[Crasher508]

Generate xml-files for:

• user retransfer ("Rücküberweisung")
• invoice payment

Why? Our new bank sometimes loses the payment reference in manual transfers. This error does not occur with an uploaded transfer.

[codecov]

Codecov Report

❌ Patch coverage is 98.83721% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.07%. Comparing base (8e3170a) to head (7e5a109).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
web/blueprints/finance/__init__.py	96.42%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #801      +/-   ##
===========================================
+ Coverage    87.89%   88.07%   +0.18%     
===========================================
  Files          280      280              
  Lines        17426    17380      -46     
===========================================
- Hits         15317    15308       -9     
+ Misses        2109     2072      -37     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.