Allow custom domains

[RobertAlbus]

BaseGa4mp is hard coded to

        self._base_domain = "https://www.google-analytics.com/mp/collect"
        self._validation_domain = "https://www.google-analytics.com/debug/mp/collect"

but this could be configurable to allow library users to send messages to MP-compatible endpoints

https://www.google-analytics.com could be configurable via ctor or a method, with google analytics being the sensible default.