Adds custom config to show/hide host usage info

[pinksi]

Objective

This PR includes the changes to add a custom configuration to show or hide the Host CPU and Host Virtual Memory information (Issue: #209) . It is added as a config parameter, and can be set as:
c.ResourceUseDisplay.show_host_usage=False (if you want to hide the host usage info, default is set as True)

How to test

Follow the set up process from contributing.md or use the following commands:

# Create a virtual environment using tool that you like. I used conda.
conda create -n jupyter-resource-usage -c conda-forge python

# activate the environment (conda, pipenv)
conda activate jupyter-resource-usage

# install the package in development mode
python -m pip install -e ".[dev]"

# link your development version of the extension with JupyterLab
jupyter labextension develop . --overwrite

# go to the labextension directory
cd packages/labextension/

# Rebuild extension Typescript source after making changes
jlpm run build

# Watch the source directory in one terminal, automatically rebuilding when needed
jlpm run watch
# Run JupyterLab in another terminal
jupyter lab --ResourceUseDisplay.show_host_usage=False

When you click the sidebar or tachometer, you should only see the Kernel usage as shown below:

When you set the show_host_usage=True, you should see all the info.

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[yuvipanda]

Thank you for opening this, @pinksi! I agree that on k8s / containerized systems, this is just a distraction that doesn't meaningfully add anything.

I don't have a full enough understanding of the TS parts, but this mostly lgtm! If nobody else reviews in a week or so, I will test locally and merge.

[jtpio]

Thanks!

Left a small inline comment, otherwise looks good!

[pinksi]

Thanks @jtpio and @yuvipanda for the review. I accidentally closed this PR, so reopened it again.

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[yuvipanda]

Thanks @pinksi for the PR, and thanks for the review @jtpio!

[jtpio]

This is now available in version 1.0.1: https://github.com/jupyter-server/jupyter-resource-usage/releases/tag/v1.0.1

[yuvipanda]

Thanks a lot, @jtpio!