feat: add custom datetime format for Cloud Storage subscriptions (#1970)

* feat: add custom datetime format for Cloud Storage subscriptions PiperOrigin-RevId: 619178723 Source-Link: googleapis/googleapis@811a857 Source-Link: https://github.com/googleapis/googleapis-gen/commit/768fb44d4daaea946a291f6795a50af26a515488 Copy-Tag: eyJwIjoiLmdpdGh1Yi8uT3dsQm90LnlhbWwiLCJoIjoiNzY4ZmI0NGQ0ZGFhZWE5NDZhMjkxZjY3OTVhNTBhZjI2YTUxNTQ4OCJ9 * 🦉 Updates from OwlBot post-processor See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md --------- Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

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