output: delayed initialization for custom loggers

output: delayed initialization for custom loggers

When a plugin is first initialized, it is too early to register
transaction loggers. Instead, a plugin can register a callback to be
called when Suricata is ready for outputs like transaction loggers to
be registered.

Likewise for library users, there is a window in SuricataInit where
transaction loggers can be registered that library users don't have
access to. So a lifecycle callback useful here as well.

Ticket #7236