Obtains the default credentials implementation to use in this environment.

Use this to obtain the Application Default Credentials for accessing Google APIs. Application Default Credentials are described in detail at goo.gl/IUuyuX.

If supplied, scope is used to create the credentials instance, when it can be applied. E.g, on google compute engine and for user credentials the scope is ignored.

@param scope [string|array|nil] the scope(s) to access @param options [Hash] Connection options. These may be used to configure

    the `Faraday::Connection` used for outgoing HTTP requests. For
    example, if a connection proxy must be used in the current network,
    you may provide a connection with with the needed proxy options.
    The following keys are recognized:
    * `:default_connection` The connection object to use for token
      refresh requests.
    * `:connection_builder` A `Proc` that creates and returns a
      connection to use for token refresh requests.
    * `:connection` The connection to use to determine whether GCE
      metadata credentials are available.