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