chore: add agent-connection-watch for workspaces (#24507)

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relates to GRU-18  
  
Adds basic implementation for Workspace Agent Connection Watch and tests.  
  
Missing are handling of logs.

coderd/httpmw/workspaceparam.go

@@ -54,3 +54,7 @@ func ExtractWorkspaceParam(db database.Store) func(http.Handler) http.Handler {
 		})
 	}
 }
+
+func WithWorkspaceParam(ctx context.Context, workspace database.Workspace) context.Context {
+	return context.WithValue(ctx, workspaceParamContextKey{}, workspace)
+}