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.
