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.

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[microsoft-github-policy-service]

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

[SteveL-MSFT]

The code below this for providerVisibilityApplied uses a case-insensitive hashtable which looks unnecessary to me? The asymmetry between this and the latter would create confusion for future people reading the code. Seems like we either need to either make them the same or add a comment why they are different.