feat(firestore): support array functions

[Linchin]

Supports the following:

• array_first
• array_last
• array_first_n
• array_last_n
• array_maximum
• array_maximum_n
• array_minimum
• array_minimum_n
• array_slice (need to double check docstring)

[gemini-code-assist]

Summary of Changes

Hello, 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 enhances the Firestore client library by integrating new array manipulation functions directly into pipeline expressions. This provides developers with more powerful and convenient ways to query and transform data within array fields, improving data processing capabilities without requiring client-side array handling.

Highlights

• New Array Functions: Introduced array_first, array_last, array_first_n, and array_last_n to the Firestore pipeline expressions, allowing users to extract specific elements or sub-arrays from array fields.

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[gemini-code-assist]

Code Review

This pull request adds support for array_first, array_last, array_first_n, and array_last_n functions in Firestore pipelines. The implementation and system tests look good. However, I found issues in the unit tests for array_first_n and array_last_n where the parameter order is asserted incorrectly, leading to internally inconsistent tests. I've provided suggestions to fix these tests.