Deprecate synchronous methods for signature calculation

[hvge]

In the future SDK release, we should deprecate the synchronous methods that calculate PowerAuth Symmetric Signatures and add asynchronous variants. This change will help to solve the following problems with the biometry:

On iOS:

• The signature calculation is a thread blocking operation if PowerAuthAuthentication object is configured for biometric signature. The asynchronous API allows us to hide all such complexity into one properly cancelable call.
• The implementation will use LAContext internally so we can fully handle all weird states, such as power button press while biometric dialog is visible.

On Android:

• The biometric signature calculation require that application has to resolve the biometry key in advance. This can be now an implementation detail hidden in asynchronous API.

I think that we need to introduce a new object, called "biometric prompt" that contain information about biometric prompt.

Related to #9, #271, #272, #495