Add `ReadOnlyProviders` option for session configuration files

[SeeminglyScience]

PR Summary

Adds option in a session configuration file to make a provider visible, but not able to be written to.

PR Context

Keeping this as WIP only because I expect issues with the test. Will take it out of draft after getting that working.

PR Checklist

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• [x] Summarized changes
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• Breaking changes
  • [x] None
  • OR
  • [ ] Experimental feature(s) needed
    • [ ] Experimental feature name(s):
• User-facing changes
  • [ ] Not Applicable
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  • [x] Documentation needed
    • [ ] Issue filed:
• Testing - New and feature
  • [ ] N/A or can only be tested interactively
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  • [x] Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • [x] I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
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  • [ ] I have considered the user experience from a tooling perspective and opened an issue in the relevant tool repository. This may include:
    • [ ] Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
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This pull request has been automatically marked as Review Needed because it has been there has not been any activity for 7 days. Maintainer, please provide feedback and/or mark it as Waiting on Author