[POC] Extensible config management

[albinsuresh]

Proposed changes

POC to extend config management capabilities of c8y-configuration-plugin with user-provided extensions.

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• [ ] Documentation Update (if none of the other choices apply)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• [ ] I have read the CONTRIBUTING doc
• [ ] I have signed the CLA (in all commits with git commit -s)
• [ ] I ran cargo fmt as mentioned in CODING_GUIDELINES
• [ ] I used cargo clippy as mentioned in CODING_GUIDELINES
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)

Further comments