[FEA] Support inferring pip packages used in the application when packaging the app

Is your feature request related to a problem? Please describe.

Although specifying dependent packages explicitly for the operator is a cleaner way to provide a reusable operator, providing automatic detection of the list of pip packages used at the application level could make users easy to use the SDK.

It would be nice to have such feature like BentoML does.

Describe the solution you'd like

Static analysis of imported code to detect pip packages used so user can choose to use it.

Instead of

@md.env(pip_packages=['monai==0.7.0', 'numpy>=1.20.1', 'SimpleITK>=2.0.2'])

User can specify

@md.env(infer_pip=True)

This is a similar API with BentoML.

Describe alternatives you've considered

Instead of implementing the feature from scratch, it is possible to use 3rdparty library to do so.

• https://github.com/bndr/pipreqs
• https://github.com/Damnever/pigar

Additional context

Requests from https://github.com/Project-MONAI/monai-deploy-app-sdk/issues/165#issuecomment-939304212