fix: Change Numbers API to make URL Form Encoded requests

[dragonmantank]

Description

Changed the POST requests of the Numbers API to use URL form encoded requests instead of JSON bodies. Also forced the Numbers API to make sure the API Key and Secret are always in the header via the correct auth type.

Motivation and Context

Looks like this was a long-standing bug with the v3 code, and when everything was transitioned to using the server-client package it was moved to normal POST requests with a JSON body instead of using form encoded requests.

Testing Details

Unit tests, and used in a demo app

Example Output or Screenshots (if appropriate)

Types of changes

• 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

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