chore: rename Function to FunctionExpression

[daniel-sanche]

Rename Function to FunctionExpression, and add a warning that other changes in the API surface may come before GA

[gemini-code-assist]

Summary of Changes

Hello @daniel-sanche, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request primarily focuses on enhancing the clarity of the Firestore Pipelines API by renaming a key class to better reflect its purpose. Additionally, it introduces explicit warnings across the relevant modules to inform users about the preview status of the API and the possibility of future breaking changes, ensuring better expectation management for developers.

Highlights

• API Renaming: The core Function class has been renamed to FunctionExpression for improved clarity and consistency within the API surface.
• Preview API Warning: Warnings have been added to several Firestore Pipelines API files, indicating that the API is currently in preview and subject to potential breaking changes in future releases.

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[gemini-code-assist]

Code Review

This pull request primarily renames Function to FunctionExpression across the Firestore pipeline-related files to better reflect its purpose. It also adds a 'Preview API' warning to the docstrings of these modules and classes. The renaming has been applied consistently. My review includes suggestions to improve code style consistency, specifically regarding indentation in the new warning docstrings and removing extra blank lines to adhere to PEP 257.