fix: Parse root certificate from either file or directory in tedge cert upload

[jarhodes314]

Proposed changes

This PR ensures the c8y.root_cert_path setting is respected in all places tedge makes an HTTP connection to Cumulocity. The setting previously existed but wasn't widely respected, meaning that operations such as retrieving a configuration file or remote access didn't work unless the Cumulocity root certificate was trusted by the system. Following these changes, it should be sufficient to set c8y.root_cert_path to point to the Cumulocity root certificate and all operations as well as commands like tedge cert upload should work.

This also fixes a separate issue with tedge cert upload where it did respect the setting, but always read the configuration from /etc/tedge/tedge.toml, rather than using the provided custom configuration directory.

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

#2951

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

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
484	0	2	484	100	1h25m35.181485s

[codecov]

Codecov Report

Attention: Patch coverage is 67.85714% with 108 lines in your changes missing coverage. Please review.

Project coverage is 78.2%. Comparing base (706cdea) to head (e637b25).
Report is 3 commits behind head on main.

Additional details and impacted files

Files	Coverage Δ
crates/common/axum_tls/src/acceptor.rs	96.4% <ø> (ø)
crates/common/axum_tls/src/files.rs	90.6% <100.0%> (ø)
crates/common/certificate/src/lib.rs	89.7% <ø> (ø)
crates/common/upload/src/upload.rs	89.1% <100.0%> (+0.4%)	⬆️
crates/core/plugin_sm/src/plugin_manager.rs	67.0% <100.0%> (+0.1%)	⬆️
crates/core/tedge/src/cli/certificate/error.rs	36.3% <ø> (ø)
...core/tedge_agent/src/file_transfer_server/actor.rs	81.7% <ø> (ø)
crates/extensions/c8y_auth_proxy/src/server.rs	89.5% <100.0%> (-0.3%)	⬇️
crates/extensions/c8y_http_proxy/src/tests.rs	93.5% <100.0%> (+<0.1%)	⬆️
...rates/extensions/tedge_downloader_ext/src/tests.rs	97.2% <100.0%> (+<0.1%)	⬆️
... and 22 more

... and 4 files with indirect coverage changes

[gligorisaev]

@jarhodes314 I created test case that should cover the bug #2951, but not being yet able to check if it will pass after installing the final build

[albinsuresh]

LGTM.