Pre-commit and PR linters and check scripts

[joniohtonen]

Added pre-commit checks/linting (ruff, yamllint, shellcheck, custom check_spdx_headers script) during commit for changed files, added workflow for running similar checks for changed fiels during PR creation.

Background

We are missing a programmatical approach for ensuring quality in our codebase. Using pre-commit to enforce certain aspects of the QC is an easy first step, as well as using a dedicated workflow for running the same checks that the PRs stay uniform in this regard too.

Goals

Enable automated, rudimentary QC when it comes to raw code and configurations files in order to avoid errors due to testing oversights etc.

Tasks

• Extend existing pre-commit to include other checks and linters than custom check_spdx_headers script.
• Add a workflow that runs these same checks when a PR is opened.

Tests

• Test linters locally
• Tested workflow by opening this PR.

Other

Majority of the existing Python and YAML files had some aspects that ruff and yamllint were opinionated about, mostly whitespace, so the pre-commit and the PR workflow only check for changed files. This is to ensure that there isn't a sudden megacommit consisting whitespace fixes etc.

[joniohtonen]

Fixed linter errors in the workflow itself.

[lauri9]

This looks like a good starting point.

Could you generate some documentation, e.g., to CONTRIBUTING.md or README.md about these checks and how to run them locally? This would help developers and agents keep up the standards set.