feat: allow starting server with bentoml.Service instance

[parano]

What does this PR address?

This PR made the following use case possible, this is especially helpful in a project development environment where no Bento was built yet.

import bentoml

# import the Service defined in `/clip_api_service/service.py` file
from clip_api_service.service import svc 

# start a server:
server = bentoml.HTTPServer(svc)
server.start(blocking=False)
client = server.get_client()
client.rank(..)

Similarly, users can also put a launch script within a service definition file and run it as a regular Python program:

# service.py
svc = bentoml.Service(...)

if __name__ == "__main__":
    server = bentoml.HTTPServer(svc)
    server.start(blocking=True)

python service.py

See discussion here: #3560 (comment)

Fixes #(issue)

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• Did you read through contribution guidelines and follow development guidelines?
• Did your changes require updates to the documentation? Have you updated
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• Did you write tests to cover your changes?

[codecov]

Codecov Report

Merging #3829 (3811528) into main (f3ef39a) will not change coverage.
The diff coverage is 0.00%.

@@          Coverage Diff          @@
##            main   #3829   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        154     154           
  Lines      12682   12736   +54     
=====================================
- Misses     12682   12736   +54     

Impacted Files	Coverage Δ
src/bentoml/__init__.py	0.00% <ø> (ø)
src/bentoml/_internal/bento/bento.py	0.00% <0.00%> (ø)
src/bentoml/_internal/service/loader.py	0.00% <0.00%> (ø)
src/bentoml/_internal/service/service.py	0.00% <0.00%> (ø)
src/bentoml/server.py	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes