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/CompletionsCapability.cs

@@ -0,0 +1,19 @@
+using ModelContextProtocol.Server;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol.Types;
+
+/// <summary>
+/// Represents the completions capability configuration.
+/// <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">See the schema for details</see>
+/// </summary>
+public class CompletionsCapability
+{
+    // Currently empty in the spec, but may be extended in the future.
+
+    /// <summary>
+    /// Gets or sets the handler for get completion requests.
+    /// </summary>
+    [JsonIgnore]
+    public Func<RequestContext<CompleteRequestParams>, CancellationToken, Task<CompleteResult>>? CompleteHandler { get; set; }
+}