Sets the function to be called when a proxy is requesting authentication. Information on the proxy in use and the security realm for the authentication are passed in the arguments [host], [port] and [realm].
The function returns a [Future] which should complete when the
authentication has been resolved. If credentials cannot be
provided the [Future] should complete with false
. If
credentials are available the function should add these using
[addProxyCredentials] before completing the [Future] with the value
true
.
If the [Future] completes with true
the request will be retried
using the updated credentials. Otherwise response processing will
continue normally.