Add clientdata accessor to session; allow access to input names via dir()

[cpsievert]

Closes #623
Closes #711

Here's an example to replicate the one on this page https://shiny.posit.co/r/articles/build/client-data/

app.py

import matplotlib.pyplot as plt
import numpy as np

from shiny import App, Session, render, ui

app_ui = ui.page_sidebar(
    ui.sidebar(
        ui.input_slider("obs", "Number of observations:", min=0, max=1000, value=500)
    ),
    ui.h3("clientData values"),
    ui.output_text_verbatim("clientdatatext"),
    ui.output_plot("myplot"),
    title="Shiny Client Data",
)


def server(input, output, session: Session):
    @render.text
    def clientdatatext():
        cdata = session.clientdata
        return "\n".join([f"{name} = {cdata[name]()}" for name in dir(cdata)])

    @render.plot
    def myplot():
        plt.figure()
        plt.hist(np.random.normal(size=input.obs()))
        plt.title("Generated in @render.plot")


app = App(app_ui, server)

TODO

• Unit test using the app provided
• Test this works with Express
• How to best document? Translate https://shiny.posit.co/r/articles/build/client-data/ to Python?