[Feature Request] Provide a base class for implementing custom IAuthorizationHeaderProvider

[jmprieur]

Is your feature request related to a problem? Please describe.
In cases where partners want to support more protocols than Bearer and Pop, and still use Microsoft.Identity.Web.DownstreamApi, and Microsoft.Identity.Web.GraphServiceClient, or Microsoft.Identity.Web.TokenAcquistion, they would want to leverage a base implementation of IAuthorizationHeaderProvider that supports Bearer and Pop, and add their own processing in derived classes

Describe the solution you'd like
Expose a new class:

namespace Microosft.Identity.Web.Extensibility
{
 public class BaseAuthorizationHeaderProvider : IAuthorizationHeaderProvider
 {
  public BaseAuthorizationHeaderProvider(IServiceProvider) {}
 }
}

This class could get hold of the ITokenAcquisistion interface (that we want to obsolete in next versions) through the service provider (locator pattern in this case to preserve the future), and new-up a DefaultAuthorizationHeaderProvider, that will remain internal.