Add pip dependencies to config

[EdwinWiseOne]

Under app_deps in the app_config.yaml, split it into edm and pip sections.

During ci build, the pip dependencies are written to a requirements.txt file, and the Dockerfile then installs pip from there using pip install -e requirements.txt.

Ensure everything works even if there are no pip dependencies.

Streamlit already installs from pip, so it is a good example of having pip installs.

[EdwinWiseOne]

A prerequesite for https://github.com/enthought/edge-product/issues/2900