feat: implement MCP elicitation support (#413)

* Add ElicitationRequest, ElicitationResult, and related types to mcp/types.go
* Implement server-side RequestElicitation method with session support
* Add client-side ElicitationHandler interface and request handling
* Implement elicitation in stdio and in-process transports
* Add comprehensive tests following sampling patterns
* Create elicitation example demonstrating usage patterns
* Use 'Elicitation' prefix for type names to maintain clarity

Author: semistrict

server/session.go

@@ -59,6 +59,13 @@ type SessionWithElicitation interface {
 	RequestElicitation(ctx context.Context, request mcp.ElicitationRequest) (*mcp.ElicitationResult, error)
 }
 
+// SessionWithElicitation is an extension of ClientSession that can send elicitation requests
+type SessionWithElicitation interface {
+	ClientSession
+	// RequestElicitation sends an elicitation request to the client and waits for response
+	RequestElicitation(ctx context.Context, request mcp.ElicitationRequest) (*mcp.ElicitationResult, error)
+}
+
 // SessionWithStreamableHTTPConfig extends ClientSession to support streamable HTTP transport configurations
 type SessionWithStreamableHTTPConfig interface {
 	ClientSession