feat: add custom datetime format for Cloud Storage subscriptions (goo…

…gleapis#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
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