Implement client for download api

[JohnPaton]

This needs cleaned up, type hinted, tested, etc. But what do you think about organizing the code like this @avaldebe? I think it's cleaner to have one fully async internal client that contains all async methods, and then have user-facing functions that use the client under the hood to do the work and wait for it to complete. The client will also handle connection sharing and resource limits.

Diff to your branch

[JohnPaton]

Ah asyncio shennanigans in the tests, I'll look into it.

[JohnPaton]

Tests are good now except existing integration tests for the old service, which are failing.

Still to do in future PRs:

• [ ] Switch cli to use new service
• [ ] Remove old service and/or swap out internals of old interface with new service + deprecation warnings

[JohnPaton]

Closing in favour of #54