Add support for checking connection status

[frederikprijck]

Changes

Adds support for the missing endpoint to check a connection's status: https://auth0.com/docs/api/management/v2#!/Connections/get_status.

Can be used:

auth0.connections.checkStatus({ id })
  .then(() => /* ONLINE */)
  .catch(() => /* OFFLINE */);

or

auth0.connections.checkStatus({ id }, function(err, result) => {
  if (err) {
    /* OFFLINE */
  } else {
    /* ONLINE */
  }
});

References

Closes #655

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