PoC: Configurable built-in bridge

[jarhodes314]

Proposed changes

Enable the built-in bridge to be configured with custom rules, ultimately to help enable a generic mapper.

At the time of creating the PR, I have implemented some parsing logic for a bridge toml template, and for testing purposes, created a main.rs in the tedge_mqtt_bridge crate which reads a given config file and debug-prints the expansion of that template. I believe it supports what we need in order to define the existing bridge rules.

For testing it out, save the following template to a file and run cargo run -p tedge_mqtt_bridge <filename>. You can play around with setting c8y.smartrest.templates and seeing the generated output change as a result. I've experimented with using serde_spanned and ariadne for contextual error reporting with invalid templates.

local_prefix = "${.c8y.bridge.topic_prefix}/"
remote_prefix = ""

[[rule]]
topic = "s/ds"
direction = "inbound"

[[rule]]
local_prefix = "c8y/" # you can override the variables within rules
remote_prefix = ""
topic = "s/us"
direction = "outbound"

[[template_rule]]
for = "${.c8y.smartrest.templates}"
topic = "s/uc/${@for}"
direction = "outbound"

[[template_rule]]
for = "${.c8y.smartrest.templates}"
topic = "s/dc/${@for}"
direction = "inbound"

TODO:

• Parse a configuration file of bridge rules
• Make the configuration variable expansion profile-aware
• Configure the bridge with said rules
• Integrate this into a mapper so it can run custom rules
• Persist the default rules from each cloud mapper like we do for flows in feat: az mapper defined using regular flows #3892
• Document the configuration format

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

• The builtin-bridge should be configurable #3894

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s. You can activate automatic signing by running just prepare-dev once)
• I ran just format as mentioned in CODING_GUIDELINES
• I used just check 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

❌ Patch coverage is 76.27119% with 126 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/extensions/tedge_mqtt_bridge/src/config.rs	84.02%	55 Missing and 22 partials ⚠️
crates/extensions/tedge_mqtt_bridge/src/main.rs	0.00%	49 Missing ⚠️

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[didier-wenzek]

Nice. Aligned with the expectation and working as described.