Add C++ stages/expressions and Constant value support

[wu-hui]

1. Add stages and expressions in C++
2. Use FSTUserDataReader to read constants

[gemini-code-assist]

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[google-oss-bot]

	1 Warning
⚠️	Did you forget to add a changelog entry? (Add #no-changelog to the PR description to silence this warning.)

Generated by 🚫 Danger

[wu-hui]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces C++ stages and expressions, along with constant value support using FSTUserDataReader. The changes touch several files, primarily in the Firestore/Source and Firestore/core directories. Overall, the changes seem well-structured and introduce important new functionality. However, there are several areas that could benefit from closer scrutiny and potential improvement.

Summary of Findings

• Redundant isUserDataRead checks: The isUserDataRead flag and associated logic in FIRConstantBridge and FIRFunctionExprBridge seem redundant. The FSTUserDataReader should handle the parsing and caching of values, eliminating the need for manual tracking. This could be simplified.
• Missing Copyright Header: The new files aggregate_expressions.h, aggregate_expressions.cc, ordering.h, and ordering.cc are missing the standard copyright header. This should be added for consistency.
• Potential Memory Leak in encodeBlob: In FSTUserDataReader.mm, the encodeBlob method copies the blob using nanopb::CopyBytesArray, but it's unclear if the original ByteString's memory is properly managed. This could lead to a memory leak if not handled correctly.
• Inconsistent use of std::move: There are some inconsistencies in the use of std::move. For example, in Where class constructor in stages.h, std::move is used for expr_, but not in CollectionSource constructor. Consistent use of std::move can improve performance.

Merge Readiness

While the pull request introduces valuable functionality, the issues identified, particularly the potential memory leak and redundant logic, should be addressed before merging. I am unable to approve this pull request, and it should be reviewed and approved by others before merging. Addressing the high and medium severity issues is highly recommended before merging.