Configure custom POST path

[erickok]

Unlike your iOS, Android (customPostPath) and web (postPath) SDKs, it seems to be impossible to override the path with a custom path. Our infrastructure follows the suggested setup where we use a custom path (instead of com.snowplow) to hopefully prevent ad/tracking blockers from intercepting the analytics calls, yet from Flutter we cannot set our own path and thus it is impossible to track. This is a blocker issue for us.

Please add support for setting the postPath/customPath from Flutter.