Refactor API response type for search

[Noam-Bendelac]

Similar to what I did for the /api/playlists/:id endpoint and its response type in f5b375c and 044bd2e, I need to have better control over what data is forwarded from the Spotify API and decide exactly what parts are needed. This will help with state management in the frontend and mocking track data for the add state.

[Noam-Bendelac]

This should include basing the GetTrackSearchResponse type on an array of TrackObjectFull (from TrackSearchResponse['tracks']['items']) instead of the broad SearchResponse type.