Feature request: Reference a `requirements.txt` file from `application.yaml`

[rudolfbyker]

It would be very useful for me to move the Python requirements from application.yaml to a requirements.txt file. Use case: I want to run something like Mypy in CI, and I want to install just the Python dependencies from application.yaml without doing a full app-builder -d.

[rudolfbyker]

I could use yq to query the requirements, but the application.yaml format is a bit cumbersome: The python version is specified in a key, so the key contains ` and.` characters.

Another spin-off feature request (breaking change) would be to redesign the application.yaml structure a bit:

Dependencies:
  python:
    version: 3.11.1
    pip: 23.1.2
    requirements:
      - foo==1.2.3
      - bar==2.*
    requirements_files:
      - requirements.txt
      - some/other/requirements.txt