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.