Add complete e-commerce flow with groups and nested flows

[Grazulex]

• Implemented a comprehensive e-commerce workflow in ecommerce-complete.yaml including user validation, payment processing, order processing, shipping, and notifications.
• Created detailed documentation for e-commerce order processing with step groups in ecommerce-order-processing.md.
• Defined reusable step groups for notifications, order processing, and user validation in separate YAML files.
• Developed GroupStep and NestedFlowStep classes to handle execution of step groups and nested flows.
• Introduced FlowGroupRegistry for managing named step groups.
• Added tests for group and nested flow functionalities, ensuring proper execution and error handling.
• Created fixtures for testing step groups and nested flows with various scenarios.
• Implemented custom steps for authentication, data validation, API calls, and more to enhance workflow capabilities.

Description

Related Issue

Closes #

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvement

Changes Made

Testing

• Tests added/updated for the changes
• All existing tests pass
• New tests pass
• Manual testing performed

Code Quality

• Code follows the project's style guidelines
• Self-review of code performed
• Code is properly documented
• No unnecessary debug code or comments left

Screenshots (if applicable)

Additional Notes

Checklist

• I have read the CONTRIBUTING.md guide
• My code follows the project's coding standards
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes