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.
