Feature/add secret type

[yuriverweij]

Add support for the Secret type (introduced in Robot Framework 7.4) to the Input Text and Input Password keywords, ensuring sensitive values can be handled securely and without accidental logging. Keeping in mind compatibility problems with older Robot Framework versions.

Secret type support and compatibility:

• Updated Input Text and Input Password keywords in formelement.py to accept both str and Secret types, extracting the value from Secret objects as needed. This ensures that secret values are not logged and are handled securely.
• Added a compatibility stand-in for the Secret type in utils/types.py, allowing the code to work with Robot Framework versions both before and after 7.4 by providing a minimal stand-in class if necessary.

Acceptance tests and helpers:

• Added new acceptance tests to textfields.robot to verify that Input Text and Input Password accept the Secret type, that plain strings are still accepted for backwards compatibility, and that secret values are not logged.
• Introduced a test tag require-rf-7.4 to skip specific Secret tests if it is not present.