Add nebar:open-proxy command for menu customization

[krassowski]

Reference Issues or PRs

A step towards nebari-dev/nebari#2217

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

Documentation

Documented in README

[github-actions]

👈 Launch a Binder on branch krassowski/jupyterlab-nebari-mode/add-menu-customizations

[krassowski]

I will merge and release a new version to make review of nebari-dev/nebari#2267 (which depends on this one) easier.