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>

src/ModelContextProtocol/Protocol/Types/TextResourceContents.cs

@@ -0,0 +1,16 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol.Types;
+
+/// <summary>
+/// Text contents of a resource.
+/// <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">See the schema for details</see>
+/// </summary>
+public class TextResourceContents : ResourceContents
+{
+    /// <summary>
+    /// The text of the item. This must only be set if the item can actually be represented as text (not binary data).
+    /// </summary>
+    [JsonPropertyName("text")]
+    public string Text { get; set; } = string.Empty;
+}