Give VPC Access Agent Service Account Permissions

[derhally]

This grants Serverless VPC Access Service Agent compute.networkUser role on subnetworks which are required for it to function.

https://cloud.google.com/run/docs/configuring/connecting-shared-vpc#grant-permissions

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[comment-bot-dev]

Thanks for the PR! 🚀
✅ Lint checks have passed.

[derhally]

@googlebot I signed it!