Add option for deterministic defaults

[giograno]

Background

For our use case, we have decided to distinguish between 3 cases when it comes to Avro compatibility:

1. Two schemas are identical;
2. Two schemas differ but are backward compatible;
3. Two schemas differ and are not compatible.

Problem

Avro supports default values, and they are often used within dataclasses.
Imagine the following code:

@dataclass.dataclasses
class PyType:
   creation_time: datetime = dataclasses.field(default_factory=lambda: datetime.now(tz=timezone.utc))

The generated schema will look like this:

{
  "type": "record",
  "name": "PyType",
  "namespace": "tests.unit.persistence.avro.aws.test_store_schemas",
  "fields": [
    {
      "type": {
        "type": "long",
        "logicalType": "timestamp-micros"
      },
      "name": "creation_time",
      "default": 1771420337157151
    }
  ]
}

Given that we use the default factories to set default values, the default will be different each time we regenerate the schema. As the code did not change, they should be identical but differ in the default.
Avro has a way to represent the schema called "canonical", which excludes the default attribute. Unfortunately, we can't use this option because any addition with a default (which is compatible) would have been marked as incompatible because the defaults are stripped.

Solution

We introduce a new option that puts a placeholder instead of factory defaults. We do this for datetime and uuids, as they are the ones we did find to change all the time.

Usage

When saving Avro states, we'll keep the option disabled, as we want to properly record defaults in the schema saved along with the records. For the schemas we want to keep on disk, we'll turn the option on.

[bentsku]

LGTM, I do not see a big issue with this, even if we ended not using it, it is behind an optional flag. We can also extend it if we find other cases.

Thanks for jumping on this, really pragmatic and clean approach 👌