Label several received messages

Applies one label change to up to 100 received messages, which may be in any of the account's inboxes. The label rules apply; sent copies have their own endpoint.

The change is all or nothing: if any ID is missing, deleted or not on this account, the request returns 404 and no message changes. On success the response has one entry per distinct ID, in request order. For more than 100 messages, send several requests.