feat(unstable): kv.watch() (#21147)
This commit adds support for a new `kv.watch()` method that allows
watching for changes to a key-value pair. This is useful for cases
where you want to be notified when a key-value pair changes, but
don't want to have to poll for changes.
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Co-authored-by: losfair <zhy20000919@hotmail.com>
