Fix async streaming with OpenAISpec

[aniketmaurya]

What does this PR do?

Old: users are forced to implement all the methods as async and not rely on default decode_request and encode_response method.

import litserve as ls

class OpenAIAPI(ls.LitAPI):
    def setup(self, device):
        pass

    async def decode_request(self, request):
        return super().decode_request(request)

    async def predict(self, request):
        for i in range(10):
            yield f"Hello, world! {i}"

    async def encode_response(self, output):
        return super().encode_response(output)

if __name__ == "__main__":
    api = OpenAIAPI(enable_async=True, spec=ls.OpenAISpec())
    server = ls.LitServer(api, accelerator="auto")
    server.run(port=8000)

New (this PR): User only implements predict and rest of the methods gets automatically converted into async.

import litserve as ls

class OpenAIAPI(ls.LitAPI):
    def setup(self, device):
        pass

    async def predict(self, request):
        for i in range(10):
            yield f"Hello, world! {i}"

if __name__ == "__main__":
    api = OpenAIAPI(enable_async=True, spec=ls.OpenAISpec())
    server = ls.LitServer(api, accelerator="auto")
    server.run(port=8000)

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Attention: Patch coverage is 75.86207% with 14 lines in your changes missing coverage. Please review.

Project coverage is 85%. Comparing base (e03ba13) to head (bc0ee32).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #552   +/-   ##
===================================
- Coverage    85%    85%   -0%     
===================================
  Files        38     38           
  Lines      2902   2940   +38     
===================================
+ Hits       2480   2504   +24     
- Misses      422    436   +14     

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