Add support for 'notification' attribute on Envelope

[e3-nbaker]

We need to customize the expiration settings for Envelopes, which are part of the 'notification' attribute. The current Envelope attribute white-list does not support this.

See:
https://developers.docusign.com/esign-rest-api/reference/Envelopes/Envelopes/create#expirations
https://www.docusign.com/blog/dsdev-trenches-reminders-expirations/