refactor(firestore): [PQ] refactor CountIf, Replace, Unnest and rename Function

[bhshkh]

1. refactor CountIf and rename Function to match Java and Node

https://github.com/googleapis/java-firestore/blob/6e30a6c11efe5d428607bfd78f82ba7b49497bd9/google-cloud-firestore/src/main/java/com/google/cloud/firestore/pipeline/expressions/BooleanExpression.java#L26-L35

https://github.com/googleapis/java-firestore/blob/6e30a6c11efe5d428607bfd78f82ba7b49497bd9/google-cloud-firestore/src/main/java/com/google/cloud/firestore/pipeline/expressions/AggregateFunction.java#L111-L121

https://github.com/googleapis/java-firestore/blob/6e30a6c11efe5d428607bfd78f82ba7b49497bd9/google-cloud-firestore/src/main/java/com/google/cloud/firestore/pipeline/expressions/FunctionExpression.java#L28

2. Revert proto files permission changes
3. Revert empty proto files
4. Refactor Replace and Unnest to match Java

https://github.com/googleapis/java-firestore/blob/6e30a6c11efe5d428607bfd78f82ba7b49497bd9/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Pipeline.java#L707-L777

https://github.com/googleapis/java-firestore/blob/6e30a6c11efe5d428607bfd78f82ba7b49497bd9/google-cloud-firestore/src/main/java/com/google/cloud/firestore/Pipeline.java#L851-L996

[gemini-code-assist]

Summary of Changes

Hello @bhshkh, 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 introduces a refactoring of the CountIf aggregate function, making its usage more explicit by requiring a BooleanExpression and relocating its method definition to the appropriate interface. Concurrently, the Function interface has been renamed to FunctionExpression to standardize naming across the Firestore client libraries.

Highlights

• Refactor CountIf Function: The CountIf aggregate function has been refactored to explicitly accept a BooleanExpression as its argument, enhancing type safety and clarity. Its corresponding method has been moved from baseExpression to the BooleanExpression interface.
• Rename Function Interface: The Function interface has been renamed to FunctionExpression to improve consistency and align with naming conventions used in other Firestore SDKs.

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[gemini-code-assist]

Code Review

This pull request refactors the CountIf aggregate function to be more type-safe by requiring a BooleanExpression, and renames the Function interface to FunctionExpression for consistency with other Firestore SDKs. The changes are well-implemented and improve the API's robustness and ergonomics. I've added a couple of suggestions to improve the clarity of the comments for the new CountIf function to better reflect its behavior.

[daniel-sanche]

LGTM