fix: Resolve optional Thumbnails when requesting PlaylistItems

[danperks]

This pull request includes several important changes to the YoutubeKit API models to enhance flexibility and robustness. The changes primarily focus on making certain fields optional and adding new fields to better represent the data structure.

Enhancements to PlaylistItem and related models:

• YoutubeKit/API/Models/PlaylistItemsList.swift: Modified the PlaylistItem struct to make snippet, contentDetails, and status optional, and reordered the fields.
• YoutubeKit/API/Models/Snippet.swift: Added new optional fields videoOwnerChannelTitle and videoOwnerChannelId to the Snippet.PlaylistItemsList struct. Also, made thumbnails optional and renamed playlistID to playlistId.
• YoutubeKit/API/Models/Thumbnail.swift: Updated the Thumbnails.PlaylistItemsList struct to make default, high, and medium optional, and added new optional fields standard and maxres.

[danperks]

Could be a dupe of #45 actually, my bad!

[Tibimac]

Could be a dupe of #45 actually, my bad!

@danperks It's not a real duplicate because your PR goes further and IHMO yours could be merged while the #45 could be canceled and closed.
Nevertheless I wonder why you renamed the playlistID property to playlistId because to keep the consistency across the project ID have to stay in uppercase.
Moreover you also left a duplicate; there is both channelId and channelID.
In order to help, I created the PR to solve those issues (danperks#3). Once you'll merge it, this PR should be updated too (I hope so ^^).

@rinov Can you please plan to merge this PR (and apply the patch from my PR above if not yet merged by @danperks in this PR) ?