mod_http_oauth2: Strip unknown extra fields from client registration We shouldn't sign things we don't understand! RFC 7591 section-2 states: > The authorization server MUST ignore any client metadata sent by the > client that it does not understand (for instance, by silently removing > unknown metadata from the client's registration record during > processing). Prevents grandfathering in of unvalidated data that might become used later, especially since the 'additionalProperties' schema keyword was removed in 698fef74ce53

Over the years, some servers stop working for various reasons, and leave
behind broken roster subscriptions that trigger failing s2s connections.
This module allows cleaning up such cases by unsubscribing local users
from their contacts on those servers. Also works for typos and the like.
Use with care.

```lua
Component "gmail.com" "unsubscriber"
```