Make FunctionAuth part of the public API

[fsenart]

It is surely an oversight, but the handy auth.FunctionAuth class is not exported.

[tomchristie]

It was a deliberate choice, since you don't need to use the class for the simple function case, you can just pass the function itself.

Eg...

def custom_auth(request):
    request["X-Auth-Token"] = "1234"

httpx.get("https://www.example.com", auth=custom_auth)

We could nonetheless expose the class, but as things currently stand it's an implementation detail, that users don't need to know about.

[tscheburaschka]

Just for the record: This class is quite useful when implementing a sequential combination of auths as prepared in httpx_auth.authentication.SupportMultiAuth. But it's fair to assume that users so deep in the rabbit hole are able to handle this case themselves.