Added custom toast component, samples, + unit tests

[OliverBack]

This PR contains adding the ability to pass a custom component to the show toast service.

• Added ShowToast overload to accept a custom component, including a combination of toast parameter and toast instance settings.
• Updated ToastServiceTests for ShowToast component methods
• Included example bUnit test.
• Added signing to Toast and ToastExtensions with a strong name including applying InternalsVisibleToAttribute for Toast.TestExtensions
• Updated Readme examples