Add STARTS_WITH/ENDS_WITH operators to StringRule

[sic2]

Add STARTS_WITH/ENDS_WITH operators to StringRule

Description

This change add a couple of operators (STARTS_WITH, ENDS_WITH) to the StringRule.

Related Issue

None

Motivation and Context

This provides more expressiveness in the rules that can be created in Regola.

How Has This Been Tested?

Change tested via comphrensive unit tests.

Types of changes

• 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 change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[jetfreakk]

Looks good, the removal of SINGLE_FACT_OPERATORS from SIngleValueRule allows more flexibility and is helpful