feat(firestore): add support for isType pipeline expression

[dlarocque]

Adds support for isType pipeline expression.

Ported from: googleapis/java-firestore#2329

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[dlarocque]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the isType Pipeline expression to Firestore, allowing users to verify the data type of fields or expressions within a pipeline. It also promotes several Pipeline-related APIs from Beta to stable by removing the @Beta annotation. The changes include new API methods, registration of the is_type function, and comprehensive integration tests. Review feedback identified a typo in the CHANGELOG's pull request reference and formatting issues in the KDoc documentation for the new isType method.

[dlarocque]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the isType Pipeline expression, allowing developers to check the data type of fields or expressions. The changes include updates to the API definition, the function registry, and the addition of comprehensive unit tests. I have provided a suggestion to improve type safety and developer experience by replacing the string-based type parameters with a structured constant object.

[milaGGL]

LGTM with a comment

[milaGGL]

LGTM