Add option for GET polling for UAID viability

[data-sync-user]

Per discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=1486010#c11 ,

It would be useful if there was an Autoendpoint URL that allowed for external UAs to poll and see if a given UAID is viable or not.

e.g. HEAD /v1/check/{uaid} which would return a JSON formatted response containing {"status":status_value} where status_value is 200 or either 404 or 410 depending on the stored UAID status. An error would be returned as a JSON formatted response containing {"error": error_code, "errno": error_number, "message": descriptive_text}, with updates to the Autopush documentation to describe these errors and resolutions.

This endpoint would need to be coordinated with the UA client team.

┆Issue is synchronized with this Jira Task

[data-sync-user]

➤ JR Conlin commented:

Since we’re returning a structured result, it makes more sense to make this a GET rather than a HEAD.
We want to return a structured result, because operational or structural causes may make a machine return a non-200 response, and we want to differentiate that from a “This UAID is no longer available” response.