Allow configuration of HttpClients used by DownstreamWebApi

[h3rmanj]

Addresses #1740

The MS docs recommends implementing resilient http requests using libraries like Polly. At the same time, the recommended way to make authenticated api calls is through the IDownstreamWebApi interface. This PR closes the gaps in the docs and allows the consumer access to the IHttpClientBuilder, allowing them to add Polly policies for their downstream apis etc.

Changed to add named HttpClients, which DownstreamWebApi then gets from IHttpClientFactory, allowing the consumer to optionally configure HttpClient per downstream service.

Example usage with this:

builder.Services.AddMicrosoftIdentityWebApi(builder.Configuration)
  .EnableTokenAcquisitionToCallDownstreamApi()
    .AddDownstreamWebApi(
      "DownstreamApi",
      builder.Configuration.GetSection("DownstreamApi"),
      httpClientBuilder => httpClientBuilder
        .AddTransientHttpErrorPolicy(builder => builder.WaitAndRetryAsync(new[]
        {
          TimeSpan.FromSeconds(1),
          TimeSpan.FromSeconds(5),
          TimeSpan.FromSeconds(10)
        })))
    .AddDownstreamWebApi(
      "AnotherApi",
      builder.Configuration.GetSection("AnotherApi"),
      httpClientBuilder => httpClientBuilder
        .AddTransientHttpErrorPolicy(builder => builder.WaitAndRetryAsync(new[]
        {
          TimeSpan.FromSeconds(1),
          TimeSpan.FromSeconds(2)
        })))
  .AddDistributedTokenCaches();

[ghost]

All CLA requirements met.

[h3rmanj]

Alternatively, if changing the API of AddDownstreamWebApi isn't desirable, I could just drop it. Multiple calls to AddHttpClient with the same client name applies all options to the same client. So we could add a named HttpClient with the serviceName internally, and the consumer could then AddHttpClient(serviceName) as well if they need it. It would be a little bit more of a hidden feature, but would work.

builder.Services.AddMicrosoftIdentityWebApi(builder.Configuration)
  .EnableTokenAcquisitionToCallDownstreamApi()
    .AddDownstreamWebApi("DownstreamApi", builder.Configuration.GetSection("DownstreamApi"))
    .AddDownstreamWebApi("AnotherApi", builder.Configuration.GetSection("AnotherApi"))
  .AddDistributedTokenCaches();

builder.Services.AddHttpClient("DownstreamApi")
  .AddTransientHttpErrorPolicy(builder => builder.WaitAndRetryAsync(new[]
  {
    TimeSpan.FromSeconds(1),
    TimeSpan.FromSeconds(5),
    TimeSpan.FromSeconds(10)
  }));
      
builder.Services.AddHttpClient("AnotherApi")
  .AddTransientHttpErrorPolicy(builder => builder.WaitAndRetryAsync(new[]
  {
    TimeSpan.FromSeconds(1),
    TimeSpan.FromSeconds(2)
  }));

[jmprieur]

@h3rmanj
Let's us get back to you on this one. We are considering having a new interface (IDownstreamRestApi) which therefore would not have breaking changes (as it would be new).

@jennyf19: let's work on collecting all the issues and PRs around IDownstreamWebApi and add the right design for IDownstreamRestApi

[h3rmanj]

@jmprieur @jennyf19
I see some work has been done in the rel/v2 branch, but I'm not sure if it's complete. What is the status of this? Will there be a chance to review before a final release?

[jmprieur]

@h3rmanj : what you wrote in #1735 (comment) would work in Microsoft.Identity.Web 2.0.5-preview (with AddDownstrreamRestApi)

[h3rmanj]

Implemented in 2.5.0