Even though most of the time these modules will be used by
connectors, there are exceptions (namely, `emqx_rule_engine`).
Besides, they are general enough to land there, more so given
that `emqx_placeholder` is already there.
This slightly changes semantics: now the attempt to create authenticator
with illegal bindings in templates will fail, instead of treating them
as literals. The runtime behaviour on the other hand should be the same.
Andrew Mayorov