Add S3 methods for sorting & filtering hierarchical tables

[edelarua]

What changes are proposed in this pull request?

• Added S3 methods tbl_sort.tbl_hierarchical() and tbl_filter.tbl_hierarchical() to sort and filter hierarchical tables.

Closes #2096

To consider:

• Do we need the .stat argument or just require users to include "n" in their table if they want to use it?
• When filtering, if there are no rows left in a section do we still keep the summary row for that section if it it meets the criteria?

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Reviewer Checklist (if item does not apply, mark is as complete)

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When the branch is ready to be merged into master:

• Update NEWS.md with the changes from this pull request under the heading "# gtsummary (development version)". If there is an issue associated with the pull request, reference it in parentheses at the end update (see NEWS.md for examples).
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