Fix Amount struct for correct url query encoding.

[VictorAvelar]

Description

Fixes the way in which an Amount object is encoded as url query params, the existing code was encoding value and currency as separate url params which was not correct for Mollie.

Motivation and context

Library bug fix.

How has this been tested?

• Unit tests added / updated
• The tests run on docker (using task test)
• The required test data has been added / updated

If you are running the tests locally, please specify:

• OS: macos Sonoma
• Go version 1.23.x

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• 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 change)
• This change requires a documentation update

Checklist

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• If my change requires a change to the documentation, I have updated it accordingly.

[VictorAvelar]

The code is correct, this fix is not needed.