Feature request: generated-sources should include resolved avsc files

[mpataki]

In order to achieve reusability in avro schemas, we have a couple options. We can use schema references on the .avsc level or we can use .avdl or .avpr, among possibly others.

Ultimately, the avro-maven-plugin resolves any schema references/dependencies and produces a single avro schema definition that is placed in it's generated java classes.

I propose that the plugin also write these resolved avro schema to self-contained .avsc in target/generated-sources/avsc/**, or similar.

This would be helpful as it would allow consumers of this plugin to then use these self-contained avro schema in down stream systems directly, rather than needing to pluck them out of the generated java classes.