Implement rule-level warning_threshold

[kapitoshka438]

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It would be great to be able to override warning threshold for a specific rule. If threshold was specified for a rule, its violations would not be counted for the main threshold and throw an error only in terms of its own limit.

In our projects we set warning_threshold to 0 to prevent any violations of included rules. We also regularly include new rules and temporarily increase the warning_threshold to allow violations for the the rule we are working on until we completely fix it everywhere. But this allows violations of other rules to appear.

[SimplyDanny]

There has been some ongoing work in #3421 for support of a "baseline" functionality. Seems like a better and more flexible solution for your request than various configured thresholds. Unfortunately, there is no active work on this feature at the moment, but at least a lot of interest.