Overhaul of Resources (#125)

* Supporting resource templates with read resource callback

Fixes #123

* Aligning the resource data structures with the 2024-11-05 schema

- Introducing separate classes for Text and Blob ResourceContents
- Custom JSON converter to allow those types to be returned, making it easier to follow the type system against the spec
- Updated tests

* Apply suggestions from code review

Co-authored-by: Stephen Toub <stoub@microsoft.com>
Co-authored-by: Peder Holdgaard Pedersen <127606677+PederHP@users.noreply.github.com>

* Putting files back where they belong

* Adding an internal constructor to prevent overloads

* Addressing feedback

* Apply suggestions from code review

Co-authored-by: Stephen Toub <stoub@microsoft.com>

* Update src/ModelContextProtocol/Protocol/Types/ResourceContents.cs

* Addressing feedback

---------

Co-authored-by: Stephen Toub <stoub@microsoft.com>
Co-authored-by: Peder Holdgaard Pedersen <127606677+PederHP@users.noreply.github.com>

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -349,7 +349,7 @@ public class McpServerTests : LoggedTest
                     {
                         return Task.FromResult(new ReadResourceResult
                         {
-                            Contents = [new() { Text = "test" }]
+                            Contents = [new TextResourceContents() { Text = "test" }]
                         });
                     },
                     ListResourcesHandler = (request, ct) => throw new NotImplementedException(),
@@ -364,7 +364,9 @@ public class McpServerTests : LoggedTest
                 var result = (ReadResourceResult)response;
                 Assert.NotNull(result.Contents);
                 Assert.NotEmpty(result.Contents);
-                Assert.Equal("test", result.Contents[0].Text);
+
+                TextResourceContents textResource = Assert.IsType<TextResourceContents>(result.Contents[0]);
+                Assert.Equal("test", textResource.Text);
             });
     }