Subquery

[cherylEnkidu]

feat: add support for Pipeline subqueries

This PR introduces support for Pipeline Subqueries, allowing users to perform complex data transformations and aggregations on subcollections within a pipeline.

Key Features

• Subcollection Pipelines: Added Pipeline.subcollection(String path) to initiate a pipeline on a subcollection of the current document.
• Variable Definition: Added Pipeline.define(...) to bind values to variables for reuse within the pipeline scope.
• Expression Enhancements:
  • Expression.currentDocument(): Reference the current document in a pipeline stage.
  • Expression.variable(String name): Access defined variables.
  • Pipeline.toArrayExpression() and Pipeline.toScalarExpression(): Convert subquery results into array or scalar expressions for use in parent pipelines.

Testing

• Added ITPipelineSubqueryTest.java containing integration tests for subquery functionality.
• Verified compatibility with the Enterprise backend.

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• proto-google-cloud-firestore-v1/src/main/java/com/google/firestore/v1/DocumentProto.java
• proto-google-cloud-firestore-v1/src/main/java/com/google/firestore/v1/Value.java