Add errhint_internal()

Add errhint_internal()

We have errmsg_internal(), errdetail_internal(), but not errhint_internal().

Sometimes it is useful to output a hint with already translated format
string (e.g. because there different messages depending on the condition). For
message/detail we do that with the _internal() variants, but we can't do that
with hint today.  It's possible to work around that that by using something
like
  str = psprintf(translated_format, args);
  ereport(...
          errhint("%s", str);
but that's not exactly pretty and makes it harder to avoid memory leaks.

Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/ym3dqpa4xcvoeknewcw63x77vnqdosbqcetjinb2zfoh65k55m@m4ozmwhr6lk6