feat(clientdata): id is now optional on clientdata.output_*() methods

[cpsievert]

See the changes to CHANGELOG for a description of what this PR adds.

A minimal example:

import matplotlib.pyplot as plt
import numpy as np

from shiny.express import render, session, ui

ui.input_dark_mode()

@render.plot
def myplot():
    plt.figure()
    plt.hist(np.random.normal(size=100))  # type: ignore

    bg_color = session.clientdata.output_bg_color()
    plt.title(f"bgcolor: {bg_color}")

[schloerke]

I like this approach!, but I feel it is in the wrong place.

👀 for a different location.

[cpsievert]

IIRC, we ended up not finding a better place?