[Flutter] Remove GetIt and make async initialization invisible to the user

[Tiagoperes]

Currently, we use GetIt to make dependency injection. We should not impose the usage of GetIt to our users. For this reason, we must find another way to initialize and provide Beagle services.

Furthermore, the initialization of Beagle is asynchronous, which is very annoying. We can't use anything Beagle related before the service is initialized. For now, the user must use BeagleSdk.openScreen, which implicitly awaits for Beagle to be ready. If the user doesn't want to call BeagleSdk.openScreen, he/she must call await BeagleServiceLocator.allReady() manually. We must find a way to make this problem inexistent or invisible for both us and the user.

[uziasferreirazup]

migrate to private repo