feat(firestore): Add ifNull and coalesce expressions

[milaGGL]

Ported from firebase/firebase-js-sdk#9753

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[github-actions]

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[milaGGL]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for ifNull and coalesce pipeline expressions in Firestore. The changes include updates to the public API, implementation of these expressions as both static and instance methods in the Expression class, and the addition of integration tests to verify their behavior. A minor typo was identified in the documentation for one of the ifNull overloads.

[yvonnep165]

A minor version bump in the gradle properties file may be required to fix the Metalava SemVer Check error. But maybe merging the latest changes from the main can solve this if someone already did that.

[yvonnep165]

LGTM!