Add sample for Spanner ADK integration

[sararob]

This includes a sample showing how to use the ADK's Spanner integration. The sample includes 3 agents:

• A basic agent for summarizing your Spanner instances and databases
• An agent that does semantic reasoning using a vector embedding column
• An agent that restricts access to specific tables in your Spanner instance

[gemini-code-assist]

Code Review

This pull request introduces a sample healthcare application demonstrating how to build SQL, semantic search, and secure database agents using the Agent Development Kit (ADK) and Cloud Spanner. It includes setup scripts, database schemas, and three distinct agent implementations. The review feedback highlights a critical need to validate the GOOGLE_CLOUD_PROJECT environment variable across all agents to prevent runtime failures when it is missing. Additionally, in the semantic agent, it is recommended to instantiate the GenAI Client at the module level rather than on every function call to avoid unnecessary overhead.