[avro] Ignoring built-in annotations

[cowtowncoder]

(moved from FasterXML/jackson-dataformat-avro#25)

In using this it seems the mapper is ignoring standard annotations that comes with avro. Fields marked with AvroIgnore are not ignored and it seems everything unions with null making the nullable annotation useless in the schema generation. Those are the two big ones I've witnessed but I'm sure the others do not work either.

[cowtowncoder]

(comment by @jmoney8080)

Here is a test proving that the process is ignoring @AvroIgnore: https://github.com/jmonette/jackson-dataformat-avro/commit/2c35da63f8a66f02bc4411a5e462c40e6dacc4b1

This test is hung off of master.

[cowtowncoder]

Dup of #14, fixed for 2.9.0.