docs(secretmanager): add sample code for receiving a Pub/Sub message

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[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/consume_event_notification.py:22, tag secretmanager_consume_event_notification

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CLAs look good, thanks!

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CLAs look good, thanks!

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CLAs look good, thanks!

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CLAs look good, thanks!

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CLAs look good, thanks!

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CLAs look good, thanks!

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CLAs look good, thanks!

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