VariantType and SecurityPolicyType should be IntEnum

[dakotahorstman]

Describe the bug

Attempts to serialize an object with SecurityPolicyType or VariantType attributes fail. Enums cannot be serialized. Changing them to IntEnum makes them serializable, plus they are already effectively IntEnums based on their implementation (i.e. named_attribute = int).

To Reproduce

import json
from asyncua.ua import VariantType, SecurityPolicyType

json.dumps({"variant": VariantType.Boolean, "security_policy": SecurityPolicyType.None})

>>> `TypeError: Object of type VariantType is not JSON serializable`
OR
>>> `TypeError: Object of type SecurityPolicyType is not JSON serializable`

Expected behavior

Serialization to succeed.

Version

Python-Version:3.12
opcua-asyncio Version (e.g. master branch, 0.9): 1.1.0