Refactor Bluem class and related components for improved readability and consistency

- Adjusted visibility and formatting of properties and methods in the Bluem class.
- Enhanced error handling in the BluemConfigurationValidator for missing environment configuration.
- Updated the Webhook class for consistent property visibility and formatting.
- Added unit test for creating a PaymentBluemRequest in BluemPaymentsTest.
- Refactored BluemTest to use createStub instead of createMock for better test clarity.
- Removed unused helper methods in BluemTest to streamline the test class.

Author: DaanRijpkema