Create a unified abstraction for RuleConfig

[nathanjmcdougall]

Motivation
Two main parts to this:

• A class for RuleConfig which documents selections and ignores, both managed and unmanaged
• Refactoring get_associated_ruff_rules method to get_rule_config, which will resolve Consider splitting get_associated_ruff_rules into two cases for managed and non-managed rules #499.

Also, we should add convenience functions in use_ruff to reduce the complexity of that function.

This will help work towards #561.

Summary of desired enhancement
Create a unified abstraction for RuleConfig