Add support for an additional address line

[Thorry84]

Added support for an additional address line in addresses, this is often required for international shipments and can be useful for NL shipments.

Description

Added a new property to address, in line with DHL naming and existing code.

Motivation and context

This code adds a missing feature to the API which DHL provides.

How has this been tested?

Created a label with an extra line, checked to new line was submitted to DHL.

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)

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.
• I have added tests to cover my changes.
• All new and existing tests passed.
• If my change requires a change to the documentation, I have updated it accordingly.
• My code follows the code style of this project.

[mvdnbrk]

Thanks!

[Thorry84]

You're welcome!