Support for copilot-generated summaries in quick info. (On-the-fly docs)

[spebl]

This introduces a new setting C_Cpp.onTheFlyDocsEnabled to control the display of the option to show copilot-generated summaries in the hover tooltip.
When enabled, and also authenticated with the vscode-copilot extension, the hover tooltip will display an option to generate a summary of the symbol with copilot.
The setting is defaulted to default which will check the feature flag control to determine if the feature should be enabled, which allows for slow rollout and the ability to rollback should any issues arise.

Updating the vscode requirement to 1.90.0 to support using copilot features with "vscode.lm".

The IntelliSense client changes supporting this feature are included in a separate PR against that repository.

Some workarounds were needed to support icon rendering in hover markdown and updating hover content dynamically which currently requires the hover to be closed and reopened. Potential fixes from proposals and unreleased patches are linked where applicable.

All feedback is very welcome!

[benmcmorran]

Left a few wording comments but mostly looks good to me ✨. Leaving official sign-off for cpptools owners.

[sean-mcmanus]

@spebl Did you want this in our next 1.22.x or what release?

[spebl]

@spebl Did you want this in our next 1.22.x or what release?

Not currently targeting a specific release, no need to block anything on this going in. I first wanted to get feedback on this approach, along with the upgrading of the vscode version. I'm taking a look now at how we can best keep support for the older versions while also using the new language model apis when available.

[benmcmorran]

Capturing notes from a quick discussion I had with @Colengms. He's going to go ahead and move from LSP-based hover to a HoverProvider which should make it easier for us to opt-in to icons and avoid async work during settings initialization. We also discussed a potential alternative using multiple HoverProviders where a provider that actually displays Copilot results simply blocks until the user clicks the Copilot link from the first provider. FYI @spebl

stale