Break PowerAuthSDK dependency on shared configurations

[hvge]

Currently, our iOS PowerAuthSDK class internally depends on several shared configuration instances: PA2ClientConfiguration.sharedInstance() & PA2KeychainConfiguration.sharedInstance(). We should break this dependency and allow object instantiation with using custom configuration objects.

Benefits

• I'm working on LimeAuth library where I would like to allow instantiate high level object with all application provided configurations. Right now, it's kind of confusing when developer has to provide own PowerAuthConfiguration and then access shared instances for networking, etc...
• Better thread safety. Later changes to shared configurations doesn't affect already instantiated SDK objects.
• Better object testability

Proposed changes

• Both, PA2ClientConfiguration and PA2KeychainConfiguration should implement NSCopying protocol.
• Add new read-only keychainConfiguration property to PowerAuthSDK. The property should return copy of internally used configuration, to do not allow post-inistantiation changes in keychain configuration.
• Add new read-only httpClientConfiguration property to PowerAuthSDK. The property should return copy of internally used configuration, to do not allow post-inistantiation changes in client configuration.
• Add new designated initializer to PowerAuthSDK to allow construct object with exact keychain & client configurations. Due to backward compatibility, we should provide fallbacks to shared config instances once one of the new parameters is nil
• Fix PA2PrivateHttpTokenProvider which also depends on PA2ClientConfiguration.sharedInstance()