Extendable multicurrency compatibility

[dpanta94]

All Submissions:

• Does your code follow the Extendables standards?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changes proposed in this Pull Request:

No specifc related issues, but PR aims to allow 3rd party developers to add externally to this plugin compatibility with more muticurrency plugins.

How to test the changes in this Pull Request:

1. Install a 3rd party multiccurency plugin not supported internally by the plugin
2. Add compatibility creating another plugin (Guide will be provided)
3. Amazon Pay plugin should detect the compatible instance.

Other information:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changelog entry

Add - Extrandalbe Multicurrency compatibility

[alexiglesias31]

LGTM!