mitmproxy addon: refactor the callback addon

[kegsay]

Whilst debugging duplicate OTK upload tests, I noticed that some requests were being tarpitted when they should not have been.

It turns out this was due to the callback addon itself not handling concurrent request/responses very well. To fix this, use aiohttp as per https://github.com/mitmproxy/mitmproxy/blob/main/examples/addons/nonblocking.py

This also adds tests which just test the callback addon itself, which will eventually fix #3.

--- PASS: TestCallbackAddon (17.06s)
    --- PASS: TestCallbackAddon/works (0.02s)
    --- PASS: TestCallbackAddon/can_be_filtered_by_path (0.03s)
    --- PASS: TestCallbackAddon/can_be_filtered_by_method (0.02s)
    --- PASS: TestCallbackAddon/can_be_filtered_by_access_token (0.03s)
    --- PASS: TestCallbackAddon/can_be_filtered_by_combinations_of_method_path_and_access_token (0.02s)
    --- PASS: TestCallbackAddon/processes_callbacks_concurrently (1.02s)

Longer term, the status_code addon will be obsoleted by the callback addon because it will sprout the ability to modify the HTTP response.

[andybalaam]

One question but if you're happy with it, looks good.