feat(config): add config to not display non configured providers

[julienrbrt]

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

Given this feature is tiny, I thought opening a discussion was not needed.
I was slightly annoyed to see the list of the non configured providers there, and given Copilot is auto configured, I couldn't simply redefine it in the config.

Before:

After:

[charmcli]

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[julienrbrt]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.