Adds documentation for the experimental JSON schema-based function declaration feature to `function-tools.md`.
Includes:
* An explanation of the `ADK_ENABLE_JSON_SCHEMA_FOR_FUNC_DECL` environment variable.
* CLI instructions for enabling and disabling the feature across macOS, Linux, and Windows (CMD & PowerShell) using tabbed code blocks.
* A Python code snippet demonstrating how to explicitly configure the feature in-code.
Zyan
a2ff9f1ae6