Allow handler to optionally be async when MockTransport is used with AsyncClient

[tomchristie]

Prompted by #1401 (comment)

Allow usages like...

async def hello_world(request):
    return httpx.Response(200, text="Hello, world!")

transport = httpx.MockTransport(hello_world)

async with httpx.AsyncClient(transport=transport) as client:
    response = await client.get("https://www.example.com")
    assert response.status_code == 200
    assert response.text == "Hello, world!"