feat: Accept database_id parameter in Firestore composite index creation

[fkwfung]

Description

Fixes #5169

This PR adds support for custom Firestore database IDs when creating composite indexes, enabling users to work with non-default Firestore databases.

Changes

• Added optional database_id parameter to FirestoreDb.__init__() (positioned after project_id)
• Updated create_collection_indexes() to accept and pass database_id
• Modified _create_composite_indexes() to use database_id in the parent_path construction
• Updated _get_or_create_collection() to pass database_id through

Key Features

• Backward Compatible: Defaults to "(default)" when not specified
• Low Risk: All changes are additive with sensible defaults
• High Value: Enables multi-database Firestore configurations

Testing

The implementation follows the exact specification from issue #5169. All existing cookbook examples continue to work as they use keyword arguments.

Example Usage

# Using default database (existing behavior)
db = FirestoreDb(project_id="my-project")

# Using custom database (new feature)
db = FirestoreDb(
    project_id="my-project",
    database_id="my-custom-db"
)

Checklist

• Code follows the project's coding standards
• Changes are backward compatible
• Documentation (docstrings) updated
• Related issue linked ([Feature Request] Accept Database ID in Firestore Composite Index Creation #5169)