This PR greatly improves our handling of remote threads in the sidebar.

One primary issue was that many parts of the sidebar were only looking
at a thread's path list and not its remote connection information. The
fix here is to use `ProjectGroupKey` more consistently throughout the
sidebar which also includes remote connection information.

The second major change is to extend the MultiWorkspace with the ability
to initiate the creation of remote workspaces when needed. This involved
refactoring a lot of our remote workspace creation paths to share a
single code path for better consistency.

Release Notes:

- (Preview only) Fixed remote project threads appearing as a separate
local project in the sidebar

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Co-authored-by: Anthony Eid <anthony@zed.dev>
Co-authored-by: Anthony Eid <hello@anthonyeid.me>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>