Add elicitation support for server-to-client user input requests (#228)

- Add ElicitAction enum (accept/decline/cancel)
- Add schema definitions for elicitation fields (string, number, boolean, enum)
- Add ElicitationSchema wrapper for requestedSchema
- Add ElicitRequest and ElicitResult for elicitation/create method
- Update ClientCapabilities to support elicitation capability
- Add elicit() method to ClientGateway
- Store client capabilities in session during initialization
- Add comprehensive unit tests
- Add elicitation example server with book_restaurant, confirm_action, and collect_feedback tools

composer.json

@@ -59,6 +59,7 @@
       "Mcp\\Example\\Server\\ClientCommunication\\": "examples/server/client-communication/",
       "Mcp\\Example\\Server\\ClientLogging\\": "examples/server/client-logging/",
       "Mcp\\Example\\Server\\CombinedRegistration\\": "examples/server/combined-registration/",
+      "Mcp\\Example\\Server\\Elicitation\\": "examples/server/elicitation/",
       "Mcp\\Example\\Server\\ComplexToolSchema\\": "examples/server/complex-tool-schema/",
       "Mcp\\Example\\Server\\Conformance\\": "examples/server/conformance/",
       "Mcp\\Example\\Server\\CustomDependencies\\": "examples/server/custom-dependencies/",