Migrate document/URI metadata tracking from workspace state to global state

https://github.com/confluentinc/vscode/pull/1595 sets up initial metadata persistence (CCloud org, compute pool info) for individual documents on a per-workspace basis, but we clear workspace state on extension activation, so re-opening the workspace will require re-setting compute pool metadata.

Once we're happy with the short-term behavior, we should move this to global state since it isn't wiped between activations and will also allow cross-workspace metadata lookups.