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

[lovelydinosaur]

Prompted by #1401 (comment)

Allow usages like...

asyncdefhello_world(request):
returnhttpx.Response(200, text="Hello, world!")
transport=httpx.MockTransport(hello_world)
asyncwithhttpx.AsyncClient(transport=transport) asclient:
response=awaitclient.get("https://www.example.com")
assertresponse.status_code==200assertresponse.text=="Hello, world!"