Return 412 if encountering a ':' in the `offset`

[data-sync-user]

https://sentry.io/organizations/mozilla/issues/3286812377/?project=6255069

We’re seeing a number of requests from iOS that contain ':' in the offset query parameter. It is not known what is originating these requests, but it was proposed that returning a 412 may break whatever loop these clients are in.

┆Issue is synchronized with this Jira Task

[data-sync-user]

➤ JR Conlin commented:

https://github.com/mozilla-services/syncstorage-rs/pull/1347 ( https://github.com/mozilla-services/syncstorage-rs/pull/1347|smart-link )