admin: add plugin lock coordination

weed/admin/plugin/lock_manager.go

@@ -0,0 +1,7 @@
+package plugin
+
+// LockManager provides a shared exclusive lock for admin-managed detection/execution.
+// Acquire returns a release function that must be called when the protected work finishes.
+type LockManager interface {
+	Acquire(reason string) (release func(), err error)
+}