refactor: update 0x quotes to be Index API quotes

[janndriessen]

For the sake of speed a lot of the quote flow still involves naming around 0x (and also some properties). Once we serve everything from the new Index Quote API, we should rename to those types and clean up the code accordingly.

TODOs

• update quote type to Index
• variable renaming (away from 0x)
• update quote results variables (maybe wait with this as the quote api might abstract even more things)
• ...