add factory flag to openbb-api launcher

[deeleeramone]

1. Why?:

   • This PR modifies openbb-platform-api to:
     • Allow a factory function to serve the FastAPI instance to the launcher.
     • Allow arbitrary app instance name via --name kwarg.
     • Use the CWD when the path to the target file is not absolute (path beginning with "/")
     • Use Uvicorn-style module references for --app and --name`.
       • openbb-api some_file:main --factory

2. What?:

   • As described above.
   • Also fixes the issues in Small bugfix on launch args #7028 where looking for a kwarg containing "_", instead of "-".

3. Impact:

   • More flexibility and out-of-the-box compatibility with FastAPI applications.
   • Less keystrokes required to launch.

To load a pre-existing widgets.json file, and skip reading openapi.json, do something like:

openbb-api --widgets-path . --no-build 

Where "." is defining the path to widgets.json as the current working directory.