Add support for GroupBy

[JKamsker]

This pull request introduces comprehensive support for LINQ-style GroupBy queries in LiteDB, enabling users to group query results using strongly-typed expressions and work with IGrouping<TKey, TElement> results. The changes span the query interface, query pipeline, expression visitor, and type mapping to provide a seamless and efficient grouping experience.

Key changes:

LINQ GroupBy Support and Interface Updates

• Added a strongly-typed GroupBy<K>(Expression<Func<T, K>> keySelector) method to the ILiteQueryable<T> interface, allowing users to group results and receive IGrouping<K, T> objects. The Select methods were also updated to return ILiteQueryable instead of ILiteQueryableResult for consistency. [1] [2] [3] [4]

Grouping Implementation

• Introduced the LiteGrouping<TKey, TElement> class as a concrete implementation of IGrouping<TKey, TElement>, enabling materialization of grouping results.
• Added BsonMapper.RegisterGroupingType to handle serialization and deserialization of groupings, ensuring correct mapping between BSON and .NET groupings.

Query Pipeline and Execution Enhancements

• Refactored the query pipeline to support grouping, including proper ordering before grouping, handling of Having, and in-memory ordering of grouped results. Added new internal structures (GroupSource, GroupedResult) and logic for efficient grouping and ordering. [1] [2] [3] [4]
• Updated query optimization to allow ORDER BY after grouping and to avoid redundant ordering when the grouping expression matches the index.

Expression Visitor and Resolver Improvements

• Enhanced the LINQ expression visitor to recognize and correctly resolve IGrouping<,> types, enabling support for LINQ methods such as Select, Where, and aggregation functions on groupings. [1] [2] [3]
• Introduced GroupingResolver to map LINQ methods and properties on IGrouping to LiteDB expressions.

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These changes collectively enable robust, efficient, and type-safe group-by queries in LiteDB, aligning its LINQ capabilities more closely with standard .NET query patterns.