Current Signer abstraction does not make it easy to use a custom Signer

[stiiifff]

For a test project, I needed to submit extrinsic to a pallet that implemented a signed extension.
The current Signer abstraction (ISigner) does not make it easy (or possible) to:

• Create a custom ISigner implementation
• Override the hard-coded AdditionalSignersUncheckedExtrinsic dictionary
• Use a custom ISigner implementation in the Application instance

Proposed improvement:

• Make the AdditionalSignersUncheckedExtrinsic method virtual (and maybe all public methods ?)
• Make the setter of Signer property in the Application class protected instead of private
• Add a CreateSignerInstance virtual method in the Application class and call it in constructor
  This would allow developers to easily create their own implementation of the ISigner & IApplication interfaces and use it in their apps.