python: Add support for struct enums

[svix-mman]

API to use struct enums

from svix.api import IngestSourceIn, CronConfig
# with fields
source_in = IngestSourceIn(
    name="name",
    type="cron",
    config=CronConfig(content_type="asd", payload="asd", schedule="asd"),
)
# without any fields
source_in = IngestSourceIn(name="name", type="generic-webhook", config={})

# read field
if isinstance(res.config, CronConfig):
    print(source_in.config.payload)