(De)serializer from/to JSON

[Kriskras99]

Original issue #449 by @PookieBuns

Currently we only support (de)serializing from/to JSON via the TryFrom<serde_json::Value> for avro::types::Value and TryFrom<avro::types::Value> for serde_json::Value implementations.
These implementations are not according to the spec:

#[test]
fn avro_rs_518_union_int_is_json_int_value() {
    let v = Value::Union(1, Box::new(Value::Int(42)));
    let json: JsonValue = v.try_into().unwrap();
    assert_eq!(json, json!({"int": 42}));
}
#[test]
fn avro_rs_518_union_enum_is_json_enum_name_value() {
    let v = Value::Union(1, Box::new(Value::Enum(5, "Problematic")));
    let json: JsonValue = v.try_into().unwrap();
    // The Value::Enum doesn't contain the enum name (some_name) so can't do the correct thing
    assert_eq!(json, json!({"some_name": "Problematic"}));
}

Both these tests will fail. The first test can be fixed using the TryFrom implementations, but the second test cannot be fixed as avro::types::Value does not contain the necessary information for named types.

If we decide to support the JSON encoding of Avro, I suggest the following:

1. Create a new module json that will contain the from_reader(reader, schema)/to_writer(writer, schema, value).
2. Create a new serializer and deserializer that wrap around serde_json's serializer and deserializer, using the schema to call the right functions on serde_json's serializer and deserializer.

• The alternative is (de)serializing to serde_json::Value first, and then implementing our serializer and deserializer around that. However, this is really slow.
• If there is a usecase for (de)serializing to serde_json::Value, support can always be added later with a to_value/from_value function (it can reuse the logic of SchemaAwareRecordFieldDefault for the serialisation part).