This commit decouples the emqx_rule_engine application from the
emqx_ee_schema_registry application by making it possible to register a
callback module that defines extra rule engine SQL functions instead of
calling a module in emqx_ee_schema_registry directly from the
emqx_rule_engine application.
Part of https://emqx.atlassian.net/browse/EMQX-9251
This ports part of the Schema Registry app from 4.x to 5.0. Here,
only support for Avro is added. Subsequent PRs will follow to add
support for other formats.
Kjell Winblad