docs: Add ADK Agent Analytics Advanced Capabilities Design Document

This design document outlines three advanced capabilities for the
ADK BigQuery Agent Analytics Plugin:

1. Trace-Based Evaluation Harness - Automated evaluation of agent
   behavior using stored traces with trajectory matching, LLM-as-judge,
   and deterministic replay for debugging.

2. Long-Horizon Agent Memory - Context and memory management using
   historical trace data including session memory, episodic memory,
   and user profile building.

3. BigQuery AI/ML Integration - Leveraging BigQuery's ML functions
   for embedding-based search, anomaly detection, and batch evaluation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: caohy1988