feat: allow restricting filesystem walk to specific folders

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Description

This change adds a new OnlyDirs parameter to the walker to only analyze the files located in a specified set of folders.

This can greatly improve the performance when a specific set of analyzers is used and the required paths for these analyzers are known.

This new parameter is analogous to the existing SkipDirs and SkipFiles parameters, but works the opposite way.
The paths follow the same syntax rules as the existing SkipDirs and SkipFiles parameters.

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Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
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