feat: implement default retrievers

tests/StevanFreeborn.Atproto.IdentityResolver.Tests/Unit/DefaultDidDocumentRetriever.cs

@@ -0,0 +1,26 @@
+using RichardSzalay.MockHttp;
+
+using StevanFreeborn.Atproto.IdentityResolver.Retrieval;
+
+namespace StevanFreeborn.Atproto.IdentityResolver.Tests.Unit;
+
+public sealed class DefaultDidDocumentRetrieverTests : IDisposable
+{
+  private readonly MockHttpMessageHandler _mockHttpMessageHandler = new();
+  private readonly DefaultDidDocumentRetriever _sut;
+
+  public DefaultDidDocumentRetrieverTests()
+  {
+    _sut = new(_mockHttpMessageHandler.ToHttpClient());
+  }
+
+  public void Dispose()
+  {
+    _mockHttpMessageHandler.Dispose();
+  }
+
+  [Fact]
+  public async Task GetByWellKnownOrThrowAsync_WhenResponse()
+  {
+  }
+}