Upgrade Avro union type serialization

[ttypic]

Right now union type value serializes into JsonObject that includes all possible types, e.g.

{
  "union_value": "string_value"
}

will be serialized into:

{
  "union_value": {
    "string": "string_value",
    "double": null,
    "long": null
  }
}

for union type ["long", "double","string"]

┆Issue is synchronized with this Jira Story by Unito