Support binary data in comm messages via buffers

[seeM]

Addresses #3974 and #4173.

This PR plumbs buffers from jupyter-adapter messages through to the main thread and specifically to our IPyWidgets notebook renderer. This unlocks a bunch of widget types (thanks to @isabelizimm for these examples):

ipywidgets.Image

import ipywidgets
with open('image.png', 'rb') as f: image = f.read()  # you'll need an actual image path
display(ipywidgets.Image(value=image, format='png'))

ipympl (matplotlib's interactive backend)

Note: Save doesn't work yet. We'll need to handle the clicked-data-url webview message in our NotebookOutputWebview.

%matplotlib ipympl
import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
x = np.linspace(0, 2*np.pi, 100)
y = np.sin(3*x)
ax.plot(x, y)

ipydatagrid

import pandas as pd
from ipydatagrid import DataGrid
data = pd.DataFrame({"A": [1, 2, 3], "B": [4, 5, 6]}, index=["One", "Two", "Three"])
w = DataGrid(data, selection_mode="cell", editable=True)
display(w)
# Now change the value of a cell, then run:
w.data
# It should reflect the updated value.
# The next line should update the value in the widget frontend:
w.set_cell_value("A", "One", 3)

bqplot

There appears to be a console warning here, we can address that in a follow-up.

import bqplot.pyplot as bplt
import numpy as np

x = np.linspace(-10, 10, 100)
y = np.sin(x)
axes_opts = {"x": {"label": "X"}, "y": {"label": "Y"}}

fig = bplt.figure(title="Line Chart")
line = bplt.plot(
    x=x, y=y, axes_options=axes_opts
)
bplt.show()

[seeM]

Before this change, ipympl outputs would end up using a simple image/png renderer.

[seeM]

This wasn't used anymore.

[seeM]

It didn't seem to be the case in my testing.

[nstrayer]

Looks good to me, but I am not familiar enough with the architecture decisions to pass judgement on the impact of changing the message structure nesting levels.

[seeM]

The smoke tests are failing because I missed a few more places affected by this change... Working on it.

[jmcphers]

LGTM, and the examples worked well in my testing.

[isabelizimm]

this fixes the problem observed in #4172 where Positron will crash once before ipydatagrid loads 🎉

[seeM]

Thank you for the reviews! I believe I've addressed all of the comments.