Avro maps are unsupported

[ayax79]

I hit this when working with a schema that defines the following:

    {
      "name": "metadata",
      "type": "map",
      "values": "string",
      "doc": "List of metadata items."
    }

From io/avro/read/schema: 85

 AvroSchema::Map(_) => todo!("Avro maps are mapped to MapArrays"),

It looks like this was last touched by @jorgecarleitao by 10/2021.

I can take a look at implementing it.. Are there any gotchas I should be aware of?