[Avro] Silently ignores complex unions

[walterschell]

Describe the bug, including details regarding any error messages, version, and platform.

arrow-go/arrow/avro/schema.go

Line 142 in c5f0943

if n.schema.(*avro.UnionSchema).Nullable() {

Only processes simple nullable unions and does not process complex unions, but silently ignores them.
Expectation:

1. Limitation to simple nullables is documented
2. By default, schema with complex unions errors out during conversion unless an optional flag to override this behavior and drop complex union types is passed.

Component(s)

Other

[zeroshade]

Thanks for reporting this, would you be willing to file a PR for a fix? Otherwise I can try to get to this next week

[walterschell]

I'm not familiar with the code base so whenever you get to it is fine.