Add support for user-defined manifest types

[nint8835]

With the old, untyped JSON/dict based manifests, users of the library could just insert whatever keys they wanted and add their own behaviour for them. With the new TOML/pydantic based manifests, this is no longer possible. Should add functionality so users of the library can provide their own custom base type or something like that, so they can cleanly add their own properties to the manifest.