protocols/workspace: Set ext workspace id for pinned workspace

The `id` is defined to be sent only once, on creation of the handle or
later. And only for workspaces that are "likely to be stable across
multiple sessions".

Set we add an `id` initially for pinned workspaces, and add one when the
workspace is pinned.

The `id` is not supposed to be human readable, so we just use a random
value.

cosmic-comp-config/src/workspace.rs

@@ -43,5 +43,6 @@ pub struct OutputMatch {
 pub struct PinnedWorkspace {
     pub output: OutputMatch,
     pub tiling_enabled: bool,
-    // TODO: name, id
+    pub id: Option<String>,
+    // TODO: name
 }