feat: allow to pass a method as token

[KillianHmyd]

Changes

This allow to have custom management of the token. For example when scaling an application horizontally we might want to share the tokens between all the application instances.
This allow to do so with the following :

const auth0ManagementClient = new ManagementClient({
  token: () => Promise.resolve('my-token')
  clientId,
  domain,
  audience,
  scope,
});

References

Following this issue : #792

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

• This change adds unit test coverage
• This change adds integration test coverage

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[adamjmcgrath]

Thanks @KillianHmyd

[adamjmcgrath]

lgtm - thanks @KillianHmyd!