Add codegen check to CI

[turtletongue]

Resolves #239

Synopsis

Code generation is being done manually and it can lead to merging out-of-sync Dart bindings.

Solution

A new codegen check is added to CI that runs make cargo.gen and makes sure that there were no file changes during this process.

Checklist

• Created PR:
  • In draft mode
  • Name contains issue reference
  • Has type and k:: labels applied
  • Has assignee
• Before review:
  • Documentation is updated (if required)
  • Tests are updated (if required)
  • Changes conform code style
  • CHANGELOG entry is added (if required)
  • FCM (final commit message) is posted or updated
  • Draft mode is removed
• Review is completed and changes are approved
  • FCM (final commit message) is approved
• Before merge:
  • Milestone is set
  • PR's name and description are correct and up-to-date
  • All temporary labels are removed

[turtletongue]

FCM

Add `codegen` check stage to CI (#240, #239)