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/ResourceTemplate.cs

@@ -8,7 +8,7 @@ namespace ModelContextProtocol.Protocol.Types;
 /// Represents a known resource template 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 ResourceTemplate
+public record ResourceTemplate : Annotated
 {
     /// <summary>
     /// The URI template (according to RFC 6570) that can be used to construct resource URIs.
@@ -33,10 +33,4 @@ public record ResourceTemplate
     /// </summary>
     [JsonPropertyName("mimeType")]
     public string? MimeType { get; init; }
-
-    /// <summary>
-    /// Optional annotations for the resource template.
-    /// </summary>
-    [JsonPropertyName("annotations")]
-    public Annotations? Annotations { get; init; }
 }