AddDownstreamWebApi support or corresponding method

### Describe the solution you'd like
For OBO flows there's an excellent extension method when an API calls another API, `AddDownstreamWebApi`. However, even though gRPC clients use HttpClient internally, this extension doesn't seem to work. It would be great to get a similar extension, or same, to work with gRPC clients in ASP.NET Core.

### Describe alternatives you've considered
Today we're using `AddCallCredentials` with code for getting the token manually using `ITokenAcquisition`.

### Additional context
Referred scenario: [A web API that calls web APIs](https://learn.microsoft.com/en-us/azure/active-directory/develop/scenario-web-api-call-api-overview)
Current solution: [Bearer token with gRPC client factory](https://learn.microsoft.com/en-us/aspnet/core/grpc/authn-and-authz?view=aspnetcore-6.0#bearer-token-with-grpc-client-factory)