feat: load external RdfBasedValidator for AspectModelValidator

[andreas-wirth]

Description

Addition to #1017. Enable ESMF-SDK to also load externally provided RdfBasedValidators

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

[github-actions]

Release v2.16.0 addresses this.