feat: add --allow-unicode flag for Unicode character handling

[bensi94]

Description

Adds a new --allow-unicode flag to control Unicode character handling in YAML output:

• CLI flag: --allow-unicode (default: False) - preserves Unicode characters instead of escaping them
• Python API: allow_unicode parameter in ConfigProcessor.process()
• Config merger: --allow-unicode flag support in himl-config-merger

Includes comprehensive tests (11 new tests covering multiple languages) and documentation with practical examples.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvement

Testing

• Tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested the changes manually

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

Additional Notes

I may not have full context of this project's design decisions. If this approach doesn't align with the project's direction, I'd be grateful for feedback or suggestions for a better alternative. Thank you for maintaining this project! 🙏

[bensi94]

@amuraru or @dobrerazvan

Could you take a look when you have a chance? 🙏

[amuraru]

let's keep this reformat out of the PR to reduce the number of changes

[bensi94]

Thanks. Fixed now.

[amuraru]

same here

[bensi94]

Thanks. Fixed now.

[amuraru]

please see my cosmetic change request above - otherwise looks good to me. Thank you for your contribution!

[bensi94]

Thanks, comments have been addressed.

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