[backend] Export injects (JSON) (#2291)

[antoinemzs]

Proposed changes

• New endpoints to export injects as JSON with attached documents

Related issues

• Contributes to Ability to import/export injects #2291

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality
• For bug fix -> I implemented a test that covers the bug

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...