Support schemas expressed as array

[gaborbernat]

Thanks for the library. The schema specification at https://avro.apache.org/docs/current/spec.html#schemas states:

A Schema is represented in JSON by one of:

A JSON string, naming a defined type.

A JSON object, of the form:
{"type": "typeName" ...attributes...} where typeName is either a primitive or derived type name, as defined below. Attributes not defined in this document are permitted as metadata, but must not affect the format of serialized data.

A JSON array, representing a union of embedded types.

avro-rs does not support JSON array format though as the following small example demonstrates:

use avro_rs::Schema;

fn main() {
    let raw_as_record = r#"
    {
      "type": "record",
      "name": "root",
      "fields": [
        {
          "name": "element",
          "type": [
            "null",
            {
              "type": "record",
              "name": "Magic",
              "fields": [{"name": "value", "type": "long"}]
            }
          ]
        }
      ]
    }
    "#;

    let raw_as_array = r#"
    [
      {
        "type": "record",
        "name": "Magic",
        "fields": [{ "name": "value", "type": "long"}]
      },
      {
        "type": "record",
        "name": "root",
        "fields": [{ "name": "element", "type": [ "null", "Magic"]}]
      }
    ]
    "#;

    let schema = Schema::parse_str(raw_as_record).unwrap();
    println!("{:?}", schema);

    let array_schema = Schema::parse_str(raw_as_array).unwrap();
    println!("{:?}", array_schema);
}

Record { name: Name { name: "root", namespace: None, aliases: None }, doc: None, fields: [RecordField { name: "element", doc: None, default: None, schema: Union(UnionSchema { schemas: [Null, Record { name: Name { name: "Magic", namespace: None, aliases: None }, doc: None, fields: [RecordField { name: "value", doc: None, default: None, schema: Long, order: Ascending, position: 0 }], lookup: {"value": 0} }], variant_index: {Null: 0, Record: 1} }), order: Ascending, position: 0 }], lookup: {"element": 0} }
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ParseSchemaError("Unknown type: Magic")