[REQ] Post process VERSION, and openapi.yaml from python-flask

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

While using the code generator for python-flask, I am unable to control how the .openapi-generator/VERSION, and <src>/openapi/openapi.yaml are formatted

Describe the solution you'd like

A documented behaviour to use the <LANG>_POST_FROCESSING_FILE environment variable to control the formatting.

Describe alternatives you've considered

• Running the formatting on all files afterwards, regardless of which files where changed

Additional context

The reason for this, is that I'd like the generated code to be generated automatically when the specification changes, in a pre-commit hook.
Additionally, the generated code must comply with the style guide of the project.

Start by locating the python-flask generator and its existing post-processing configuration, then trace how it handles .openapi-generator/VERSION and /openapi/openapi.yaml. Document and implement the requested _POST_FROCESSING_FILE behavior for these files, and verify that generated output can be formatted selectively for pre-commit use.