Migrate data classes to Pydantic-based models

[nwunderly]

We want to make our data classes in models.py compatible with Pydantic in order to facilitate better interoperability with API and database use cases.

What would need to be done:

• Add pydantic as a dependency
• Make DiscordObject a subclass of pydantic's BaseModel class
• Implement pydantic-compatible type hints in all data classes
• Ensure that all data classes in that file are compliant with pydantic's type checking
• No breaking changes to the actual library should be necessary for this

Happy Hacktoberfest! :smile: