Proposed changes

This adds support for a writable device ID to aid @rina23q's work. I'm marking this as a refactoring as it doesn't really perform the task in itself, although I have modified tedge_config.rs to use the new feature for verifying it actually works as intended.

Still TODO:

• Support a default value, so c8y.device.id defaults to device.id
• Make the device ID logic actually do something useful
• Check the test names are appropriate
• Make config.device.id() private

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• 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