Use different name for baseUrl argument in Frontegg constructor

[mmaitlen]

Since the argument baseUrl is the name as the reference global variable on line 32 of the Frontegg class, the global variable baseUrl isn't being set and a Null Pointer Exception is thrown later on in the sign in workflow.

https://github.com/frontegg/flutter-sdk/blob/main/lib/frontegg_class.dart#L32