[BUGFIX]--Address mem leak in ConfigValue destructor for strings.

[jturner65]

Motivation and Context

This addresses a memory leak in the ConfigValues where non-trivially constructible objects (like strings) were never getting properly disposed of.

Valgrind output before fix :
==22314== LEAK SUMMARY: ==22314== definitely lost: 418,522 bytes in 4,963 blocks

after fix :
==28380== LEAK SUMMARY: ==28380== definitely lost: 13,544 bytes in 42 blocks

How Has This Been Tested

Locally c++ and python tests pass, valgrind's mood definitely improved.

Types of changes

• Docs change / refactoring / dependency upgrade
• 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

• 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 completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.