Update media type and JSON handling in OpenAISpec

[fabigr8]

To comply with actual OpenAI API streaming responses i changed the content type and the json handling in the streaming implementation of openAI-spec see #359

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs? --> Not applicable
• Did you write any new necessary tests? --> not necessary

How does this PR impact the user?
As a user, I want to use LitServe to connect to third-party libraries or applications (such as Open WebUI) via openAI streaming API. This PR changes the LitServe OpenAI-spec to fully comply with the OAI streaming API and solve existiing connection issues.

What does this PR do?

Fixes #359

• passed 128/129 Tests.
• test: "e2e" - "test run with port" wouldn't run, probably bc of restricted write access (not sure)
• retest with own code (see bug: OpenAI-Spec should send text/event-stream #359 ) worked.

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

Did you have fun?

YES 💯

[aniketmaurya]

Awesome, thanks for the PR @fabigr8! Added a small comment.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96%. Comparing base (719e037) to head (54177b4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #360   +/-   ##
===================================
  Coverage    96%    96%           
===================================
  Files        22     22           
  Lines      1408   1408           
===================================
  Hits       1345   1345           
  Misses       63     63