Adds a "missing" status to the "agency" specification.

[wellorder]

Explain pull request

This PR creates a missing status as part of the agency specification. It is often the case that scooters go missing for a short period of time due to GPS issues, connectivity issues, etc. Under the current spec, this is supposed to be handled by deregistering the scooter and setting its state to removed, but this makes it difficult to distinguish scooters which have been removed by our operations people and scooters which have simply gone missing, which causes confusion when someone comes across a scooter in the wild which shows up as removed. What's more, deregistering and re-registering a scooter is more work to implement than changing the state to and from missing would be.

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).

• Yes, breaking

As written, this removes the missing transition to removed, which would break existing implementations that use this transition.

Impacted Spec

Which spec(s) will this pull request impact?

• agency

Additional context

This follows up discussion in #67 and #376 (comment) . Mark said he would work on such a PR, but since I had already started on this one I figured I could post it. I was holding off until I had figured out provider changes, but I think he was right and that can be put in a separate PR.

[marie-x]

Thanks @wellorder !

[marie-x]

Propose we fold this into the Agency/Provider reconciliation project #271

[CLAassistant]

All committers have signed the CLA.

[thekaveman]

missing was included as an event_type with #506. Thanks for the work on it @wellorder!