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/ToolsCapability.cs

@@ -0,0 +1,42 @@
+using ModelContextProtocol.Protocol.Messages;
+using ModelContextProtocol.Server;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol.Types;
+
+/// <summary>
+/// Represents the tools capability configuration.
+/// <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">See the schema for details</see>
+/// </summary>
+public class ToolsCapability
+{
+    /// <summary>
+    /// Gets or sets whether this server supports notifications for changes to the tool list.
+    /// </summary>
+    [JsonPropertyName("listChanged")]
+    public bool? ListChanged { get; set; }
+
+    /// <summary>
+    /// Gets or sets the handler for list tools requests.
+    /// </summary>
+    [JsonIgnore]
+    public Func<RequestContext<ListToolsRequestParams>, CancellationToken, Task<ListToolsResult>>? ListToolsHandler { get; set; }
+
+    /// <summary>
+    /// Gets or sets the handler for call tool requests.
+    /// </summary>
+    [JsonIgnore]
+    public Func<RequestContext<CallToolRequestParams>, CancellationToken, Task<CallToolResponse>>? CallToolHandler { get; set; }
+
+    /// <summary>Gets or sets a collection of tools served by the server.</summary>
+    /// <remarks>
+    /// Tools will specified via <see cref="ToolCollection"/> augment the <see cref="ListToolsHandler"/> and
+    /// <see cref="CallToolHandler"/>, if provided. ListTools requests will output information about every tool
+    /// in <see cref="ToolCollection"/> and then also any tools output by <see cref="ListToolsHandler"/>, if it's
+    /// non-<see langword="null"/>. CallTool requests will first check <see cref="ToolCollection"/> for the tool
+    /// being requested, and if the tool is not found in the <see cref="ToolCollection"/>, any specified <see cref="CallToolHandler"/>
+    /// will be invoked as a fallback.
+    /// </remarks>
+    [JsonIgnore]
+    public McpServerPrimitiveCollection<McpServerTool>? ToolCollection { get; set; }
+}