Some prep work for further changes to the executor

[dmjb]

These changes will simplify some upcoming PRs. They include:

1. Moving the rule type engine into its own sub package.
2. Removing the rule type PB struct from the evaluation parameters. The
   alerting code currently accesses the rule type information by
   multiple paths (some of it is passed to the constructor for the
   alert, and other bits from the evaluation parameters). By picking a
   single way to pass the rule type around, changes for caching are
   simplified.

Changes validated locally.

Summary

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Fixes #(related issue)

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• Bug fix (resolves an issue without affecting existing features)
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Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
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[coveralls]

coverage: 53.057% (+0.002%) from 53.055%
when pulling 47ecdc8 on cache-cleanup-prep
into 8324246 on main.

[blkt]

Non-blocking comment.

[blkt]

suggestion (non-blocking): I would name it simply rules, this package is under internal/engine anyway.

[dmjb]

@blkt Selecting a good name for this package was a pain. The problem is that rules, rule, ruletype and ruletypes are common variable names in the code which import this package. ruleengine was considered, but that is just annoying to type :)