Use extension tools for getting an API v2 token

[sandrinodm]

The current implementation includes logic to get an API v2 token.

The extension tools already contains logic to fetch the token and also to cache it for the lifetime of the token (eg: if the token is valid for 5 minutes, the token will only be cached for 5 minutes).

This will also give us less code to maintain.

[sandrinodm]

Example: https://github.com/auth0-extensions/auth0-extension-tools#auth0-management-api