Dynamically add state vars

[picklelo]

Currently all state vars need to be defined as fields within the State class. Sometimes it is useful to dynamically add vars after the state class is already defined.

Minimally this requires:

• logic to set all the relevant state var info: https://github.com/pynecone-io/pynecone/blob/main/pynecone/state.py#L114:

        for prop in cls.base_vars.values():
            if not utils._issubclass(prop.type_, utils.StateVar):
                raise TypeError(
                    "State vars must be primitive Python types, "
                    "Plotly figures, Pandas dataframes, "
                    "or subclasses of pc.Base. "
                    f'Found var "{prop.name}" with type {prop.type_}.'
                )
            cls._set_var(prop)
            cls._create_setter(prop)
            cls._set_default_value(prop)

• adding it as a Pydantic field

Then we should be able to do something like

State.add_var(pc.BaseVar(name="myvar", type_="str"))

[Lendemor]

I'll have a look at this issue.

Since I have a use case that would benefit from it, it should make testing it a bit easier.