feat: Add elicitation support to MCP protocol

Implement elicitation capabilities allowing servers to request additional information
from users through clients during interactions. This feature provides a standardized
way for servers to gather necessary information dynamically while clients maintain
control over user interactions and data sharing.

- Add ElicitRequest and ElicitResult classes to McpSchema
- Implement elicitation handlers in client classes
- Add elicitation capabilities to server exchange classes
- Add tests for elicitation functionality with various scenarios