Overhaul McpClientFactory/McpServerFactory (#87)

Handlers are now specified before connecting the client/server. Otherwise, race conditions exist. Handlers move into being a part of the McpClientOptions/McpServerOptions, so that they're provided to the factories. Where relevant, the handlers are specified as part of the capability descriptors, so that they go hand in hand.

The factories are no longer stateful. Instead of allocating a factory and then calling a create method on it, you just call a static factory method.

src/mcpdotnet/Server/McpServerOptions.cs

@@ -1,5 +1,6 @@
 
 using McpDotNet.Protocol.Types;
+using System.Text.Json.Serialization;
 
 namespace McpDotNet.Server;
 
@@ -34,4 +35,10 @@ public record McpServerOptions
     /// Optional server instructions to send to clients
     /// </summary>
     public string ServerInstructions { get; init; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the handler for get completion requests.
+    /// </summary>
+    [JsonIgnore]
+    public Func<RequestContext<CompleteRequestParams>, CancellationToken, Task<CompleteResult>>? GetCompletionHandler { get; init; }
 }