feat: inherit all Enums from str to make JSON serialization possible

[adubovik]

The following code throws an exception, because Enums aren't JSON serializable.
One needs to inherit Enum from str to make it work.

import json
from aidial_sdk.chat_completion import Message, Role

msg = Message(role=Role.SYSTEM, content="test")
dict = msg.dict(exclude_none=True)
print(json.dumps(dict)) # exception: TypeError: Object of type Role is not JSON serializable

The pattern of converting DIAL messages to OpenAI message (which is Python dict) is common in DIAL applications.
The exception doesn't allow to do it in a straigh-forward way and one have to resort to the hack like this:

import json
from aidial_sdk.chat_completion import Message, Role

msg = Message(role=Role.SYSTEM, content="test")
dict = msg.dict(exclude_none=True)
+ dict["role"] = dict["role"].value
print(json.dumps(dict)) # ok

The PR allows to avoid this hack.