Support custom build service account

[djas19]

TL;DR

Support setting a custom service account that will be used for building the function in cloud build

Terraform Resources

https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/cloudfunctions_function#build_service_account

Detailed design

Custom build service account can enhance security and are supported on cloud build and cloud functions now. Supporting them can prove necessary for companies that do not allow usage of default service accounts.

Additional information

No response

[djas19]

Relates to PR #256

[github-actions]

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