Add meta Field to getReactionCounts Aggregation

[Silent-Watcher]

Feature: Aggregate on Meta Fields

Description:
Extend the aggregation system to allow for basic grouping and counting based on the meta field. For example, count reactions grouped by reaction and meta.source.

Acceptance Criteria:

• New method: Model.getReactionCountsByMeta(reactableId, metaFieldPath).
• Returns counts grouped by both reaction and the value at the specified metaFieldPath (e.g., meta.source).
• Example output: { like: { mobile: 5, web: 3 }, love: { web: 2 } }.
• Tests are added for this new method.
• Add usage examples in docs