Should have simple way to declare types of input vars

Currently it's possible to declare the types of input values like this:

class MyInputs(Inputs):
    txt: reactive.Value[str]
    go: reactive.Value[int]

def server(input: Inputs, output: Outputs, session: Session):
    input = cast(MyInputs, input)
    ...

But it would be nice to be able to do something like this:

class MyInputs(Inputs):
    txt: str
    go: int

Notes:

• Some inputs (like those from dynamic UI) will have a possible value of None
• This should inform Python that both input.txt and input["txt"] are of type reactive.Value[str].

I think it should be possible to do these things. TypedDict, for example, does some complex stuff with types:
https://github.com/python/cpython/blob/bd8b05395ad8877f4a065562444e117b1650c022/Lib/typing.py#L2381-L2438

Start by reading the Inputs type and reactive.Value definitions referenced in the issue, then trace how attribute and mapping access are typed. The change is complete when annotations such as txt: str and go: int expose reactive.Value[str] and reactive.Value[int] for both access forms, including possible None values for dynamic inputs.