feat(client): fix user agent information and format

[VictorAvelar]

Description

Fixes the content and format for the user-agent string to make it compliant with Mollie's requirements.

Motivation and context

Closes #291

How has this been tested?

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• [x] Unit tests added / updated
• [x] The tests run on docker (using make test)
• [ ] The required test data has been added / updated

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• OS: macos Sonoma 14.0
• Go 1.20.5

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• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] This change requires a documentation update

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• [x] If my change requires a change to the documentation, I have updated it accordingly.