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.
Fixes https://emqx.atlassian.net/browse/EMQX-9130
Since buffer workers always support async calls ("outer calls"), we
should decouple those two call modes (inner and outer), and avoid
exposing the inner call configuration to user to avoid complexity.
For bridges that currently only allow sync query modes, we should
allow them to be configured with async. That means basically all
bridge types except Kafka Producer.
Stefan Strigler