Add option to whitelist _html fields from attributes on CacheMarkdownField

What does this MR do?

CacheMarkdownField concern removes the _html fields and cached_markdown_version from attributes because they can contain unredacted HTML, which could be a security issue. This MR adds an option to whitelist which _html fields is safe to keep.

What are the relevant issue numbers?

#59208 (closed)

Does this MR meet the acceptance criteria?

/cc @stanhu @ashmckenzie @jarv

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