mod_http_oauth2: Add option for specifying TTL of registered clients Meant to simplify configuration, since TTL vs ignoring expiration is expected to be the main thing one would want to configure. Unsure what the implications of having unlimited lifetime of clients are, given no way to revoke them currently, short of rotating the signing secret. On one hand, it would be annoying to have the client expire. On the other hand, it is trivial to re-register it.

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"
```