Refactor EasyMediator with DI and full feature support

Enhanced the `EasyMediator` class with the following updates:

- Added dependency injection support via `IServiceProvider`.
- Implemented `CreateStream`, `Publish`, and `Send` methods.
- Introduced pre- and post-processing for requests.
- Added exception handling with support for handlers and actions.
- Created utility methods for async enumerable adaptation and task results.
- Replaced `NotImplementedException` stubs with full implementations.
- Improved XML documentation for better clarity and usage guidance.
- Enhanced reflection usage for dynamic handler resolution.

Author: bvdcode