Add project selector to Setup webview when there are multiple DVC projects in the VS Code workspace

[mattseddon]

If the user opens a VS Code workspace that contains multiple DVC projects both the experiments and new remotes sections of the setup webview will only display information relating to the first found DVC project. Rather than cater to this advanced scenario we should add a "selector" which forces the user to select a single project to operate on.

• To fit in with the way we handle this in the rest of the app we probably want to use a quick pick.
• We could tie in with dvc.focusedProjects if there is a single value (to get the initial value).
• We should only display this information when there are multiple DVC projects in the workspace.
• Let's not persist this information between sessions.

Maybe we can add a small focused project section to the top of the page under these circumstances. If we use a quick pick we can add a button to trigger it.

We need to remember this is an advanced scenario and unlikely to help with onboarding. IMO we should do something quick to stop breakages/confusion but do our best to not overcomplicate things.

[mattseddon]

I looked further into this:

For the experiments section, we display that experiments are set up correctly if any project in the workspace has data. The part that may give more unexpected results is the show experiments button. It will always open experiments for the first project. I can fix this by rewiring the button to use dvc.showExperiments which will prompt the user to select a project to run against.

[mattseddon]

The above PR will close this. I think we take the approach of aggregating the individual project's data from the workspace and providing pass/fail information based on that. Managing/updating information from the page should use the normal workflow of asking the user to select a project to operate on if there is > 1 project in the workspace.