sink_find_early() is a convenient function that can be used instead of
sink_find() during parsing time in order to try to find a matching
sink even if the sink is not defined yet.
Indeed, if the sink is not defined, sink_find_early() will try to create
it and mark it as forward-declared. It will also save informations from
the caller to better identify it in case of errors.
If the sink happens to be found in the config, it will transition from
forward-declared type to its final type. Else, it means that the sink
was not found in the config, in this case, during postresolve, we raise
an error to indicate that the sink was not found in the configuration.
It should help solve postresolving issue with rings, because for now only
log targets implement proper ring postresolving.. but rings may be used
at different places in the code, such as debug() converter or in "traces"
section.