feat: c8y basic auth improvements

[reubenmiller]

Proposed changes

Minor improvements to the Cumulocity Basic Authentication feature (introduced in #3196).

• Include path to the credentials file in the error message when it does not exist
• Rename credentials to credentials.toml (to make it more clear that a toml formated file is expected)
• Use config-dir aware default value for the c8y.credentials_path configuration value (to support custom install locations)

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

• Support Cumulocity IoT basic device registration to allow SmartREST 1.0 usage for older clients #3036

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

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/core/c8y_api/src/http_proxy.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

📢 Thoughts on this report? Let us know!

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
528	0	2	528	100	1h34m37.915263s

[didier-wenzek]

Approved