feat: add Sheets custom function with ADK AI agent

[PierrickVoulet]

Description

Adding a new code sample that showcases how to integrate an AI agent in a Google Sheets custom function.

Is it been tested?

• Development testing done

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have performed a peer-reviewed with team member(s)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules