feat: export PipelineWatcher (#1742)

`PipelineWatcher` is the construct used in delivlib to monitor CodePipeline pipelines. It is very useful and we can leverage it to monitor pipelines that aren't managed by delivlib (e.g OpsPipeline). 

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lib/index.ts

@@ -16,3 +16,4 @@ export * from './pull-request';
 export * from './chime-notifier';
 export * from './pipeline-notifications';
 export * from './package-integrity';
+export * from './pipeline-watcher';