Merge HTTPTransport into the client

Follow-up of !241 (merged)

The arkindex.client.transports abstraction layer is not necessary. It was designed to allow using other libraries and protocols to make API requests, like using WebSockets or using anything other than requests for HTTP(S). We don't need any of this; the HTTPTransport class could be merged into the API client and simplified significantly.

This is probably better done after both #41 (closed) and #43 (closed).