feat: added isOn must-variant or default

[petruki]

This pull request introduces new "must-variant" methods to the Switcher API Go SDK, which simplify error handling by allowing default values to be returned when errors occur. The documentation and tests have been updated to cover these new methods and their usage.

New must-variant methods for error handling:

• Added IsOnOrDefault and IsOnWithDetailsOrDefault methods to the Switcher struct, allowing callers to specify default values that are returned if an error occurs. These methods also notify the client's error callback when available.

Documentation updates:

• Updated README.md to document the new must-variant methods, including usage examples and error handling guidance. [1] [2]

Testing improvements:

• Added new tests in switcher_test.go to verify the behavior of the must-variant methods, including cases for both valid and invalid switcher configurations and error callback notifications.
• Added a test for validation errors when the switcher is invalid.
• Imported the time package to support new test scenarios.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud