refactor: Rules class helper methods as free functions

[MichaelThamm]

Issue

We have some static methods in Rules that we want to move to free functions so that they can be reused in other rules-like classes e.g., SigmaRules.

Solution

Checklist

• I have added or updated relevant documentation.
• PR title makes an appropriate release note and follows conventional commits syntax.
• Merge target is the correct branch, and relevant tandem backport PRs opened.
• This change warrants a release, so I have updated the project.version field in the pyproject.toml file.

Context

These static methods were private so we have relative confidence that they were not used externally.

Ref to other PRs for context:

• feat: Sigma rules #196
• feat: Sigma rules pt.2 charmlibs#467

Testing Instructions

Use these free functions in the Rules class which is thoroughly tested by the existing unit tests in cosl.

Upgrade Notes