Fix a memory leak in HTML::Pipeline::SanitizationFilter::WHITELIST

What does this MR do?

Fix the leak mentioned in 504a3b5e by converting the methods that returns lambdas into class ones. Please see commit message for the why.

Are there points in the code the reviewer needs to double check?

N/A

Why was this MR needed?

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?