Make user field optional in embedding request

[nagar-ajay]

Describe the change
Added omitempty json tag for the User field in the EmbeddingRequest struct. This field is optional in embedding requests as per OpenAI spec.

Issue: If you don't specify any user in the embedding request, the current spec sends the user as an empty string for which embedding models throw errors.

Provide OpenAI documentation link
https://platform.openai.com/docs/api-reference/embeddings/create#embeddings-create-user

Describe your solution
Added omitempty json tag for the User field in the EmbeddingRequest struct. This will make the User field optional as per openai embeddings spec.

Tests
Tested the above change by sending embeddings requests to the embeddings model with following payload:

curl -k -X 'POST' '/embeddings' \
 -H 'accept: application/json' \
 -H 'Content-Type: application/json' \
 -d '{
      "model": "embedding-model",
      "input": "Hello world",
      "user": "test"
}'

curl -k -X 'POST' '/embeddings' \
 -H 'accept: application/json' \
 -H 'Content-Type: application/json' \
 -d '{
      "model": "embedding-model",
      "input": "Hello world",
      "user": ""
}'

curl -k -X 'POST' '/embeddings' \
 -H 'accept: application/json' \
 -H 'Content-Type: application/json' \
 -d '{
      "model": "embedding-model",
      "input": "Hello world"
}'

[sashabaranov]

@nagar-ajay thank you for the PR! Please the latest changes from master here, so the CI will pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.86%. Comparing base (774fc9d) to head (df90855).
Report is 73 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #899      +/-   ##
==========================================
+ Coverage   98.46%   98.86%   +0.40%     
==========================================
  Files          24       26       +2     
  Lines        1364     1761     +397     
==========================================
+ Hits         1343     1741     +398     
+ Misses         15       14       -1     
  Partials        6        6              

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[nagar-ajay]

Updated the PR.