Avro generator does not enforce `StreamWriteConstraints` (2.17 / 3.0.0-SNAPSHOT)

[cowtowncoder]

Unlike other codecs, work via #386 did not add full checking for Avro: this because serialization side does not use fully incremental/streaming generation: instead value objects from Apache Avro library are constructed and their serialization methods called. It would be possible to add proper nesting level handling there, and should be done, when someone has time to dig into it.
This should not be super difficult and I can help if need be.