Essentials.WebAuthenticator refactored to interface and singleton

[moljac]

Description of Change

Implements #

	public interface IWebAuthenticator
	{
		Task<WebAuthenticatorResult> AuthenticateAsync(Uri url, Uri callbackUrl);

		Task<WebAuthenticatorResult> AuthenticateAsync(WebAuthenticatorOptions webAuthenticatorOptions);
	}

Additions made

• Adds

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)
• Targets a single property for a single control (or intertwined few properties)
• Adds the property to the appropriate interface
• Avoids any changes not essential to the handler property
• Adds the mapping to the PropertyMapper in the handler
• Adds the mapping method to the WinUI, Android, iOS, and Standard aspects of the handler
• Implements the actual property updates (usually in extension methods in the Platform section of Core)
• Tags ported renderer methods with [PortHandler]
• Adds an example of the property to the sample project (MainPage)
• Adds the property to the stub class
• Implements basic property tests in DeviceTests

Does this PR touch anything that might affect accessibility?

• Does this PR introduce a new control? (If yes, add an example using SemanticProperties to the SemanticsPage)
• APIs that modify focusability?
• APIs that modify any text property on a control?
• Does this PR modify view nesting or view arrangement in anyway?
• Is there the smallest possibility that your PR will change accessibility?
• I'm not sure, please help me

If any of the above checkboxes apply to your PR, then the PR will need to provide testing to demonstrate that accessibility still works.

[moljac]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[mattleibow]

Looks really nice!

[mattleibow]

Do we still need this new things? Don't we have this 2 param api on the interface now?