Feature: support custom transport for internal httpx client

[onerandomusername]

I use a custom transport for my httpx client in order to use aiohttp internally (as we're already using aiohttp everywhere in our application). I would like to be able to affect the httpx client below in order to set custom configuration for the session. I'm also unable to provide a subclass of the httpx.Client for githubkit though I would like to.

While I am specifically concerned about the async clients, I think both the sync and async clients could benefit from this support.

[yanyongyu]

I can add a transport option to the config. You may impl the httpx.BaseTransport and httpx.AsyncBaseTransport to use your connection logic. But, since httpx uses different transport in sync/async client, we need to consider how to define this option's type.

[yanyongyu]

Custom transport can also be used in testing. By passing httpx.MockTransport to the client, we can mock the api call and return a fake response.