Add loading indicator to sidebar

[code-asher]

Depending on how long the query takes, it can look like the sidebar is temporarily broken.

[EhabY]

We can easily achieve this by fetching inside getChildren, currently, we fetch in the background (thus no loading indicator) then we refresh and getChildren just reads from memory which makes it super fast:

https://github.com/coder/vscode-coder/blob/a1ad85e03e1f92116a2771a67a3d34561d9a807c/src/workspace/workspacesProvider.ts#L241