Make MessageStatus public and add all documented enum options

[aotimme]

This change allows users of this crate access to the MessageStatus enum by adding it to the public API. It also adds a little documentation with a link to the Twilio docs (https://www.twilio.com/docs/sms/api/message-resource#message-status-values) and adds the few remaining status values from the docs.

[neil-lobracco]

Hey @aotimme , this makes sense to me (it's definitely better), but my concern is that since this is already exposed to library consumers via the status field, that this would break any users who are consuming that field, so this will need to be a breaking change.