feat: Add enhanced aggregation support

[kholdrex]

• Add aggregation methods (count, sum, avg, min, max)
• Add statistical functions (variance, stddev)
• Add database-specific group_concat function
• Add advanced stats() method for comprehensive column statistics
• Add custom_aggregation() for custom SQL aggregations
• Add convenient alias support for all aggregation methods
• Add automatic alias sanitization for complex column names
• Integrate aggregations seamlessly with existing query DSL
• Add comprehensive test suite for all aggregation features
• Update documentation with aggregation examples
• Update RBS type definitions for new methods

[Copilot]

Pull Request Overview

This PR adds comprehensive aggregation support to SimpleQuery, enhancing the library with database-level aggregation functions that provide better performance and developer experience compared to loading full record sets for calculations.

Key changes:

• Implements basic aggregation methods (count, sum, avg, min, max) and statistical functions (variance, stddev, group_concat)
• Adds advanced aggregation features including a stats method for comprehensive column statistics and custom aggregation support
• Integrates aggregations seamlessly with the existing query DSL, allowing chaining with other query methods

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
lib/simple_query/clauses/aggregation_clause.rb	Core aggregation functionality with database-specific implementations
lib/simple_query/builder.rb	Integration of aggregations into the main query builder with fluent interface
spec/simple_query/clauses/aggregation_clause_spec.rb	Comprehensive test coverage for aggregation clause functionality
spec/simple_query/builder_spec.rb	Integration tests for aggregation methods within the query builder
sig/simple_query.rbs	Type definitions for new aggregation methods
lib/simple_query.rb	Module loading for the new aggregation clause
README.md	Documentation updates with aggregation examples and usage guide
CHANGELOG.md	Release notes detailing the new aggregation features

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