Code quality improvements

[msaroufim]

📚 Documentation

Need to touch on

• [x] pre-commit #1598
• [x] Linting in CI #1645
• [x] Move to squash and merge - done in Github settings
• [x] good-first-issue - mentioned in new CONTRIBUTING.md more prominently
• [x] Revamp issue and workflow template and clarify CONTRIBUTING.md https://github.com/pytorch/serve/pull/1649
• [ ] Handler updates
  • [ ] Debugging standalone handlers https://github.com/pytorch/serve/pull/1661 - PR Ready
  • [ ] Modularity #1631 - PR Ready #1664
• [ ] Refactor tests
  • [ ] Prereq: Refactor tests into one directory - I want to run pytest . and have it just work not need any assumptions about mar files generated offline or stubs generated in some other script. We should use conftest.py for everything setup related
  • [ ] Flaky test removal #1586
  • [ ] Test time reduction #1637
• [ ] Test coverage #1636 #1665
• [ ] docstrings
• [ ] Types? #1419
• [ ] Github bot to run benchmarks, notify on regression in benchmarks, auto assignment to review