Add API endpoint to upsert subscriptions with attribs

[dfabulich]

Fixes #3133

[knadh]

Hi @dfabulich. I've only been able to give this a cursory look so far, but this should be /api/subscribers/:id/subscription and not a multi-subscriber endpoint.

[dfabulich]

I think it makes sense to do some bikeshedding on how exactly to spell the endpoint URL, but I don't think it makes sense to have an :id parameter in the URL.

The idea is that the API client has an email address and N list IDs, but the client doesn't know whether the email address is already subscribed or not, and certainly doesn't already know the ID of the existing subscriber (if any).

I'm trying to mirror the shape of /api/public/subscription's endpoint, but in an authenticated API endpoint, so I can set some attribs, and I'm specifically trying to avoid making multiple API requests.