Finished GNP Auth and SimSwap MVP implementation

[SecondeJK]

This PR adds a number of features which have been led by the addition of the first Ericsson Global Network Platform API from Vonage - SimSwap.

Description

In order to use SimSwap, a new handler for credentials has been introduced - GnpHandler and a new credentials type, Gnp. These credentials hold the registered subscriber number and extend off the Keypair object.

GNP requires an OAuth flow, which has been implemented within the GnpHandler object. Please note that a lot of this behaviour is for internals only, and should never need to be modified by the end developer.

You can find more information on what the Handler is doing here: https://developer.vonage.com/en/api/camara/auth

Motivation and Context

Evolution of Vonage SDKs and APIs

How Has This Been Tested?

New test suite added for both GNP handlers and SimSwap

Example Output or Screenshots (if appropriate):

Types of changes

• [ ] 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)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.