Add server "completions" capability (#232)

* Split apart Capabilities.cs

No code changes, just moving each type to its own file.

* Add server completions capability

src/ModelContextProtocol/Protocol/Types/SamplingCapability.cs

@@ -0,0 +1,18 @@
+using ModelContextProtocol.Protocol.Messages;
+using ModelContextProtocol.Server;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol.Types;
+
+/// <summary>
+/// Represents the sampling capability configuration.
+/// <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">See the schema for details</see>
+/// </summary>
+public class SamplingCapability
+{
+    // Currently empty in the spec, but may be extended in the future
+
+    /// <summary>Gets or sets the handler for sampling requests.</summary>
+    [JsonIgnore]
+    public Func<CreateMessageRequestParams?, IProgress<ProgressNotificationValue>, CancellationToken, Task<CreateMessageResult>>? SamplingHandler { get; set; }
+}