field_order attribute causes weird behavior

[DavidMcGurkTNG]

Describe the bug
When trying to define field_order for classes which inherit from AvroBaseModel, this can cause some weird side-effects. Field order seems to work like a private attribute, since it is created when running .avro_schema(), and this field_order created by running this method overwrites the field_order which is explicitly defined. This can also have the effect that if one pytest checks that each schema inheriting from AvroBaseModel has a complete (user defined) field order, and then a subsequent test checks that .avro_schema() successfully generates the schema, then these tests will catch the error if run in this order, but not if run in a different order.

It was also found that on >= 0.63.2, sometimes editing the field order of a child class could affect the field order of instances of the parent class.