Allow custom domains #41
Description
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.