fix(pubsub)!: make ShutdownBehavior non_exhaustive (#5533)

We want to make ShutdownBehavior non_exhaustive to support possible new
shutdown behaviors in the future. This is a breaking change, but we are
already planning to release a major version in our next change. We don't
expect this to cause many issues since it seems unlikely that users will
be matching on this enum. It is only used as the input in a single
setter.

Fixes #5532

Author: suzmue

src/pubsub/src/subscriber/shutdown_behavior.rs

@@ -14,6 +14,7 @@
 
 /// The behavior on shutdown.
 #[derive(Clone, Copy, Debug, PartialEq)]
+#[non_exhaustive]
 pub enum ShutdownBehavior {
     /// The subscriber stops reading from the underlying gRPC stream.
     ///