Update ADK doc according to issue #1227 - 7 - VertexAiSearchTool dynamic config

[adk-bot]

7. Document dynamic Vertex AI Search configuration

Doc file: docs/tools/google-cloud/vertex-ai-search.md

Current state:

Describes basic usage of VertexAiSearchTool.

Proposed Change:

Add a section "Dynamic Configuration (e.g., Per-User Filtering)".
Explain that you can subclass VertexAiSearchTool and override _build_vertex_ai_search_config(self, context).
Provide an example of setting a filter based on context.state["user_id"].

Reasoning:
VertexAiSearchTool now supports dynamic configuration via the new _build_vertex_ai_search_config method, which is useful for applying context-aware filters (like user permissions) at runtime.

Reference: src/google/adk/tools/vertex_ai_search_tool.py