feat: Add endpoint for uploading assets

[gzurowski]

Allows to create presigned URLs for uploading assets (PDF, JPEG, PNG) for comments, notes and papers.

Example request:

curl --request POST \
  --url http://localhost:8000/api/asset/upload/ \
  --header 'content-type: multipart/form-data' \
  --form filename=test.pdf \
  --form content_type=application/pdf \
  --form entity=paper

Example response:

{
  "presigned_url": "https://researchhub-dev-storage.s3.amazonaws.com/uploads/papers/users/3/b48f890a-ae44-46f7-832a-7d783f009111/test.pdf?[...]&Expires=1740507772",
  "object_key": "uploads/papers/users/3/b48f890a-ae44-46f7-832a-7d783f009111/test.pdf",
  "object_url": "https://storage.dev.researchhub.com/uploads/papers/users/3/b48f890a-ae44-46f7-832a-7d783f009111/test.pdf"
}

The presigned_url is the URL the client needs to upload the file using a PUT request.
The object_url is the final URL of the uploaded file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.82%. Comparing base (538c7eb) to head (2fe20d1).
Report is 25 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2130      +/-   ##
==========================================
+ Coverage   67.77%   67.82%   +0.05%     
==========================================
  Files         427      431       +4     
  Lines       23586    23627      +41     
==========================================
+ Hits        15985    16026      +41     
  Misses       7601     7601              

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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