refactor: add support to define_tedge_config for (optionally) multi-value fields

[jarhodes314]

Proposed changes

In order to add support multiple Cumulocity connections, we need the data model defined for tedge.toml to support multiple cloud connections. This PR adds support in define_tedge_config! for a #[tedge_config(multi)] attribute, which allows the field to store either the underlying struct directly or a hashmap of strings to structs (which provides us with multiple named configurations for a single key).

More concretely, this will allow tedge_config to support configurations like:

[c8y.edge]
url = "https://edge.example.com"
proxy.bind.port = 8001
bridge.topic_prefix = "c8y-edge"

[c8y.cloud]
url = "https://cloud.example.com"
proxy.bind.port = 8002
bridge.topic_prefix = "c8y-cloud"

[mqtt]
bridge.built_in = true

while still also supporting as it does currently:

[c8y]
url = "https://c8y.example.com"

This PR doesn't make any changes to the existing tedge_config definitions and therefore doesn't change the behaviour of thin-edge, hence the refactoring classification.

Still TODO:

• Update key parsing logic
• Update tedge config list logic
• Rename unused fields in match blocks to _
• Other TODOs left in code

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

[codecov]

Codecov Report

Attention: Patch coverage is 93.81443% with 90 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/common/tedge_config_macros/src/multi.rs	75.10%	59 Missing and 1 partial ⚠️
...ates/common/tedge_config_macros/impl/src/reader.rs	96.19%	9 Missing and 3 partials ⚠️
...rates/common/tedge_config_macros/impl/src/query.rs	99.49%	3 Missing and 1 partial ⚠️
.../tedge_config/src/tedge_config_cli/tedge_config.rs	66.66%	2 Missing ⚠️
...mon/tedge_config_macros/impl/src/input/validate.rs	77.77%	1 Missing and 1 partial ⚠️
crates/common/tedge_config_macros/src/option.rs	90.00%	2 Missing ⚠️
crates/core/tedge/src/cli/config/commands/add.rs	0.00%	2 Missing ⚠️
crates/common/axum_tls/src/config.rs	66.66%	0 Missing and 1 partial ⚠️
crates/common/tedge_config_macros/impl/src/dto.rs	93.75%	0 Missing and 1 partial ⚠️
...common/tedge_config_macros/impl/src/input/parse.rs	50.00%	1 Missing ⚠️
... and 3 more

Additional details and impacted files

📢 Thoughts on this report? Let us know!

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
505	0	2	505	100	1h33m8.767507999s

[didier-wenzek]

Approved. Thank you for this tour de force.

Despite its underneath complexity, tedge config is simple to use both by end users and developers (despite or thanks to this complexity?). However, we reached a point where things are really complicated and I would be very careful about adding additional features.

[didier-wenzek]

Suggested change

	// This key has diverged from the currrent key's parents, so empty the list
	// This key has diverged from the current key's parents, so empty the list

[didier-wenzek]

I have a preference for the former error message.