Add ability to search for comments within a post

[dessalines]

Requirements

• Is this a feature request? For questions or discussions use https://lemmy.ml/c/lemmy_support or the matrix chat.
• Did you check to see if this issue already exists?
• Is this only a feature request? Do not put multiple feature requests in one issue.
• Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.
• Do you agree to follow the rules in our Code of Conduct?

Is your proposal related to a problem?

As noted in the issue below, it'd be nice for users to be able to search for comments within a post.

Currently our SearchCombinedQuery does not allow filtering by a post_id.

Describe the solution you'd like.

Add a post_id filter to the SearchCombinedQuery, to allow this. Add unit tests to make sure its working correctly.

Describe alternatives you've considered.

NA

Additional context

Context: LemmyNet/lemmy-ui#3854 (comment)