[Feature]: Configurable sanitation rules

Feature Summary

Expose HTML sanitazion rules in the configuration

What problem does this feature solve?

HTML sanitation is extremely important and is a much welcome feature, but it would be interesting to allow the user to extend the hardcoded list of allowed tags and their attributes, either globally or (ideally) per-feed.

Proposed Solution

Have a new configuration block to extend the list of allowed tags or attributes, e.g.

p[class|style]

would authorize class and style attributes on paragraphs.

Alternatives Considered

#3846 proposed to disable sanitation altogether per-feed. This would probably also work, but that issue was closed for lack of discussion.

Additional Context

No response

Checklist

• I have searched existing issues to ensure this feature hasn't been requested before.
• I understand that feature requests are not guaranteed to be implemented.
• I agree to follow the project's contribution guidelines.