List & retrieve /taxonomies endpoint

[oguzkocer]

Follows established patterns to implement /taxonomies endpoint.

List /taxonomies endpoint has the following argument:

type Limit results to taxonomies associated with a specific post type.

However, I couldn't get this to work for any post types except for post. I believe this corresponds to the $object_type in this source function, but it's not clear to me what other values would be appropriate here.

This might mean that using PostType here might be incorrect, but for now I left the other test cases as a TODO.

@jkmassel @crazytonyli I'd appreciate your help with this one.