feat: Add subquery support in pipeline

[cherylEnkidu]

Summary

This PR implements the Query Pipeline Subquery API in the Android SDK, enabling complex data retrieval patterns including joins, aggregations on related collections, and value binding.

New API

Pipeline

• define(aliasedExpression: AliasedExpression, ...): Defines one or more variables in the pipeline's scope for internal reuse.
• define(stage: DefineStage): Overload for defining variables using a DefineStage object.
• toScalarExpression(): Converts the pipeline into an expression that evaluates to a single scalar result.
• toArrayExpression(): Converts the pipeline into an expression that evaluates to an array of results.
• subcollection(path: String): Initializes a pipeline scoped to a subcollection for use in subqueries.
• subcollection(source: SubcollectionSource): Overload for creating subcollection pipelines with a specific source.

Expression

• variable(name: String): Creates an expression that refers to a variable defined in the pipeline scope.
• currentDocument(): Creates an expression that refers to the current document being processed.

Implementation Details

• toScalarExpression and toArrayExpression handle the execution of sub-pipelines and format the results based on the output structure (unwrapping single fields or returning maps for multiple fields).
• Validations are added to ensure relative subcollection pipelines are used correctly within a parent context.

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📝 PRs merging into main branch

Our main branch should always be in a releasable state. If you are working on a larger change, or if you don't want this change to see the light of the day just yet, consider using a feature branch first, and only merge into the main branch when the code complete and ready to be released.

[wu-hui]

Couple nits on documentation, LGTM!

[wu-hui]

Approved.

[markarndt]

Approved.