[sign_in_with_apple] Replace static methods with instance methods

[klisiewicz]

Issue

The major problem with static methods is testability. Static methods are impossible to tests since they cannot be mocked by Mockito.

Right now the only way of testing code that uses SignInWithApple is to mock SignInWithApple.channel which breaks hermetisation, reveals implementation details and seems to be very low level.

Solution

Convert SignInWithApple to a singleton and change static methods to instance methods.

[HenriBeck]

Sounds like a good improvement, though we might not have immediate time for this, we are always happy to accept a PR for this.

[ivanthz]

@klisiewicz good point. Same problem here.

[vousmeevoyez]

any example ?