TBD-980 feature: Support AVRO Schema import and export to Talend MR jobs
https://jira.talendforge.org/browse/TBD-980

1. Add support for UNION type:

(a) primitives – {"type": "boolean"}

(b) but also a union of null with a primitive for isNullable types –
{"type": ["boolean", "null"]}

The isNullable metadata should be determined from whether null is in the
schema.

2. Fix an UI bug