python: copy the current Python generator into a "pydantic v1" generator

[multani]

This generator will be deprecated over time and the normal generator will focus on Pydantic v2.

@wing328 this is the fork we discussed
@krjakbrjak

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (upcoming 7.1.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

thanks for the PR.

• we will need to decouple abstract python codegen as well (which currently holds a lot of pydantic-related code)
• I'll copy the tests (petstore integration tests) into the new folder as well and update the CI to ensure pydantic v1 related samples are still tested as part of the CI workflow.
• I'll copy the unit tests of python client generator to the new one for pydantic v1

I'll merge this one first and file a new one for the above.

[wing328]

FYI @fa0311 @taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11) @tomplus (2018/10) @arun-nalla (2019/11) @krjakbrjak (2023/02)

[wing328]

UPDATE: filed a follow-up PR #16658