Add support for displaying image/png data.

[teeler]

This change maps image/png display data from execution results as tags with in-line data.

Side note - Hi! I'm Tyler, I work on Colab at Google. Huge fans of this project - we're using it to show folks how to deploy Colab notebooks on Cloud Run with the kernel gateway (https://x.com/GoogleColab/status/1864765949913739725).

I added this bit of functionality so users could display simple (and only png) objects in notebooks, for example this pair of cells:

# ResponseInfo GET /plot
print(
    json.dumps(
        {
            "headers": {"Content-Type": "text/html"},
            "status": 200,
        }
    )
)

And

# GET /plot

# prompt: Generate a random array with numpy and plot it (thanks Gemini!)

import numpy as np
import matplotlib.pyplot as plt

# Generate a random array
random_array = np.random.rand(10)

# Plot the array
plt.plot(random_array)
plt.title("Random Array Plot")
plt.xlabel("Index")
plt.ylabel("Value")
plt.show()

Produces correct output for /plot

as an in-line <img> tag.

<img alt="<Figure size 640x480 with 1 Axes>" src="data:image/png;base64,iVBORw....." >

It also has the capability of mixing between images/html/text.

Happy to re-arrange/format/etc. any of this code, thanks for reading!

[teeler]

@parente (sorry for tagging you directly) - I'd be happy to fix up some of these tests in exchange for some feedback on this PR. ;)