enable asynchronous processing in LitAPI

[aniketmaurya]

What does this PR do?

Users will be able to make async calls inside the LitAPI methods. To use async methods, users need to set the LitAPI.enable_async to True.

Note that the standard non-async way still works and async is opt-in.

import litserve as ls
+ import async_fn from some_async_lib

class SimpleLitAPI(ls.LitAPI):
-    def decode_request(self, request):
+    async def decode_request(self, request):
+        await async_fn()
        return request["input"]

-    def predict(self, x):
+    async def predict(self, x):
        return x**2

-    def encode_response(self, output):
+    async def encode_response(self, output):
        return {"output": output}

if __name__ == "__main__":
-    server = ls.LitServer(SimpleLitAPI())
+    server = ls.LitServer(SimpleLitAPI(enable_async=True))
    server.run(port=8000)

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?
• Did you write any new necessary tests?

PR review

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If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

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[codecov]

Codecov Report

Attention: Patch coverage is 34.28571% with 46 lines in your changes missing coverage. Please review.

Project coverage is 87%. Comparing base (f2237f5) to head (f8286cf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #477   +/-   ##
===================================
- Coverage    88%    87%   -2%     
===================================
  Files        37     37           
  Lines      2184   2247   +63     
===================================
+ Hits       1928   1945   +17     
- Misses      256    302   +46     

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