Add support for DelegatingHandler

[qetza]

Is your feature request related to a problem? Please describe.
Currently for using Microsoft.Identity.Web when calling APIs you have to use IDownstreamWebApi or do it manually through ITokenAcquisition. This can be complicated/time consuming for existing applications.

Describe the solution you'd like
It would be a good solution to provide a DelegatingHandler which would use ITokenAcquisition to get a token and inject it in the Authorization HTTP headers. This way you only have to update your Startup class where you register your HTTP clients.
This can also benefit for teams that prefer managing all HTTP connection transversal topics through DelegatingHandler (Polly...).

Describe alternatives you've considered

Additional context
I've already discuss this with Jean-Marc and as I have a basic implementation done I will send PR.

[jmprieur]

Thanks @qetza

The options to call an API would then become:

• If you want to use Microsoft Graph, inject GraphServiceClient
• If you want to call a REST API handling Json and don’t have an SDK, use IDownstreamWebApi
• If you are using an SDK:
  • If this is an Azure SDK, use the TokenAcquisitionTokenCredential which was introduced in 1.9
  • If this is another SDK that takes an Http delegate handler, then use a (new class to be provided) TokenAcquisitionDelegateHandler
  • If this is another SDK that requires a token, just use ITokenAcquisition.

[qetza]

Any idea when the PR will be included and released?

[jennyf19]

Included in 1.15 release