Refactor Resource/ResourceTemplate to subclass Annotated (#50)

The `Resource` and `ResourceTemplate` types should derive from a base `Annotated` type having the `Annotations` property as per the [spec](https://github.com/modelcontextprotocol/specification/blob/main/schema/2024-11-05/schema.ts#L821)

This small change makes the C# types match the schema more closely.

Also renamed Resources.cs to Resource.cs to reflect the type name.

src/ModelContextProtocol/Protocol/Types/Resource.cs

@@ -0,0 +1,34 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol.Types;
+
+/// <summary>
+/// Represents a known resource that the server is capable of reading.
+/// <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/2024-11-05/schema.json">See the schema for details</see>
+/// </summary>
+public record Resource : Annotated
+{
+    /// <summary>
+    /// The URI of this resource.
+    /// </summary>
+    [JsonPropertyName("uri")]
+    public required string Uri { get; init; }
+
+    /// <summary>
+    /// A human-readable name for this resource.
+    /// </summary>
+    [JsonPropertyName("name")]
+    public required string Name { get; init; }
+
+    /// <summary>
+    /// A description of what this resource represents.
+    /// </summary>
+    [JsonPropertyName("description")]
+    public string? Description { get; init; }
+
+    /// <summary>
+    /// The MIME type of this resource, if known.
+    /// </summary>
+    [JsonPropertyName("mimeType")]
+    public string? MimeType { get; init; }
+}