Add ADR-0018: Foundry Evals integration design

Captures the design for integrating Azure AI Foundry Evaluations with
agent-framework. Key decisions:

- EvalItem with conversation (list[Message]) as single source of truth
- query/response derived from configurable conversation split strategies
- Tools as list[FunctionTool] (including auto-extracted MCP tools)
- FoundryEvals provider with auto-detection of evaluator capabilities
- LocalEvaluator with @function_evaluator decorator for local checks
- Consistent Python/C# APIs: evaluate_agent, evaluate_workflow

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: alliscode