…eature organization

Workspace Restructuring:
- Added `rmqtt-plugins` as both workspace member and dependency
- Updated workspace members to include both directory and individual plugins
- Added `rmqtt-plugins = "0.1"` to workspace dependencies

New Meta-Package (`rmqtt-plugins` v0.1.0):
- **Unified Plugin Management**: Single package for all rmqtt plugins
- **Comprehensive Feature System**: Organized feature flags including:
  * Storage features with backend-specific sub-features
  * Core functionality plugins (ACL, HTTP API, counters, auth)
  * Bridge plugins for external system integration
  * Utility plugins (system topics, rewriting, web hooks)
  * Cluster plugins for distributed deployments

- **Enhanced Feature Hierarchy**: Added aggregate features:
  * `retainer` - enables all retainer backends
  * `message-storage` - enables all message storage backends
  * `session-storage` - enables all session storage backends

- **Documentation**: Comprehensive module docs with plugin categories
- **Selective Compilation**: Conditional compilation for all plugins

Benefits:
1. **Simplified Dependency**: Single import for all plugin functionality
2. **Modular Architecture**: Fine-grained control via feature flags
3. **Better Organization**: Logical grouping of related plugins
4. **Reduced Bloat**: Compile only needed components
5. **Improved Discoverability**: Clear documentation of available features

The changes create a structured plugin ecosystem while maintaining full backward compatibility with existing plugin usage patterns.