Add niche search methods

[devicarus]

This PR adds two methods for searching niches:

• search_niches(query) calls the /v2/niches/search endpoint to get niches matching the query
• search_niche(niche_id) calls the /v2/niches/{niche_id}/gifs endpoint to get GIFs that belong to the niche

I also propose renaming NichesInfo to NicheInfo as it represents info about a singular niche. This would make it consistent with UserInfo and TagInfo. If you agree with this change, I can include it in this PR (or submit it separately).

[scrazzz]

Hey, thanks for taking your time to implement this feature.

I've already started working on this locally. If you would have joined my Discord server and discussed about this then I would have appreciated it since I usually don't accept such large changes to the codebase. Anyhow since you've already implemented it I guess I'll merge this PR. But before that I'd like to test this code for any errors. I'm currently busy with another project so it might take me some time. But have you tested out this code yet?

[devicarus]

Thanks for taking a look!

Yes, I tested the new methods locally and they worked as expected with both the sync and async APIs. Let me know if you’d like me to adjust anything.

[scrazzz]

Alright, thanks for letting me know. I'll give you my reviews/changes soon