Support exporting query results to GCS

### Prerequisites

- [x] Search the [current open issues](https://github.com/googleapis/genai-toolbox/issues)

### What are you trying to do that currently feels hard or impossible?

Data agent produces may be sent for downstream analysis, not necessarily returned to LLM.

### Suggested Solution(s)

- Implement saving to GCS in `bigquery-execute-sql`.
- Return GCS url as the result

### Alternatives Considered

Add dry run parameter to `bigquery-execute-sql`

### Additional Details

_No response_