Standardize CLI formatting with mix format

[Ayanda-D]

Proposed Changes

Elixir formatter was introduced in version 1.6.0. This change ensures
RabbitMQ CLI formatting conforms to Elixir's standard formatting.
Format checking is added to build steps, which means new code
changes will need to run the formatter to ensure successful CLI build.

mix format is also executed for the CLI and contribution notes updated.

Types of Changes

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

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• I have read the CONTRIBUTING.md document
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• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

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