Add option to generate typed constants from DBC BA_ attributes

[felixvanoost]

Adds an option to generate typed constants from DBC BA_ attributes and place them within the existing generated message types. This is useful for DBCs that define per-message or per-signal metadata, like AUTOSAR E2E protection or SecOC parameters.

Users define a struct that their crate owns and specify where each field should come from by adding one or more AttributeStruct entries to the generator Config. I have created an example to show the syntax in the README.

[codecov]

Codecov Report

❌ Patch coverage is 95.27027% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/lib.rs	95.27%	0 Missing and 7 partials ⚠️

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