Consider brute-force protection on the Server-to-Server Sharing Endpoint

[LukasReschke]

This is a little bit a trickier one, having brute-force protection there as well would be nice. However, the way S2S is implemented right now may lead to people locking out servers by intention.

Needs some more thoughts as well as status messages so that the other Nextcloud partiall throttles. Related to #437

[szaimen]

I suppose this issue is still valid? If not, please close this issue!