[Feature Request] Implement GetAuthorizationHeaderAsync in ID.Web and use it for DowmstreamApi, GraphServiceClient,

[sruke]

What?

• Implement IAuthorizationHeaderProvider.GetAuthorizationHeaderAsync in DefaultAuthorizationHeaderProvider
• Update DownstreamApi, MicrosoftGraph, GraphServiceClient and BaseAuthorizationHeaderProvider to use the new higher level API that can create a header for on-behalf-of an app or user instead of separate APIs (GetAuthorizationHeaderForAppAsync and GetAuthorizationHeaderForUserAsync). There is no change to the existing DevEx.

Why?

• This is being done to accommodate 1P specific protocols.