Restructure IMcpEndpoint, IMcpClient, and IMcpServer (#723)

* Remove `McpEndpoint` & `IMcpClient`

* Revise naming, add abstract classes

* Fix missing renames

* Move extension methods to instance methods

* Add back old API, but obsoleted

* Add back obsoleted factory classes

* Move deprecation attributes

* `SseClientTransport` -> `HttpClientTransport`

* Update src/ModelContextProtocol.Core/Server/AugmentedServiceProvider.cs

Co-authored-by: Stephen Halter <halter73@gmail.com>

* PR feedback

* PR feedback

* Add back dispose lock

* Fix docs projects

* PR feedback

* Undo `docs/*.md` changes

---------

Co-authored-by: Stephen Halter <halter73@gmail.com>

docs/concepts/progress/samples/server/Tools/LongRunningTools.cs

@@ -10,7 +10,7 @@ public class LongRunningTools
 {
     [McpServerTool, Description("Demonstrates a long running tool with progress updates")]
     public static async Task<string> LongRunningTool(
-        IMcpServer server,
+        McpServer server,
         RequestContext<CallToolRequestParams> context,
         int duration = 10,
         int steps = 5)