Add missing/lost event to provider spec

[jiffyclub]

Explain pull request

This is a companion pull request to #429 to add a "missing"/"lost" event to the provider spec. My hope is to unblock #376 (/vehicles endpoint). I'm not at all sure I've done this correctly, so pointers appreciated.

It might also be worth noting that this event type would be changing to "unknown" in the future per @thekidder's suggestion in #271 (comment).

Is this a breaking change

A breaking change would require consumers or implementors of the API to modify their code for it to continue to function (ex: renaming of a required field or the change in data type of an existing field). A non-breaking change would allow existing code to continue to function (ex: addition of an optional field or the creation of a new optional endpoint).

• No, not breaking

This only adds a field so I don't think it's breaking, but not 100% sure on the conventions here.

Impacted Spec

Which spec(s) will this pull request impact?

• provider

Additional context

Add any other context or screenshots about the feature request here.

[jiffyclub]

Closing per discussion on the OMF Provider call today (2020-02-13), we'll skip adding missing/lost at this point since it'll be included in the overhaul of vehicle status/event type coming in 0.5.0 (see discussion in #271).