feat: Add custom service account on Bigquery subscription

[tristan-externelama-sketch]

Description

This is a copy/update of the PR #280 that seems not to be active.

This PR adds support for custom service accounts in BigQuery subscriptions, allowing users to specify a service_account_email instead of relying solely on the default Pub/Sub service agent.

Changes

• Added service_account_email optional field to bigquery_subscriptions variable
• Configured service_account_email in the bigquery_config block
• Updated bigquery integration test
• Updated README.md documentation and metadata.yaml automatically by running make docker_generate_docs

Usage Example

bigquery_subscriptions = [
  {
    name                  = "my-bq-subscription"
    table                 = "project.dataset.table"
    service_account_email = "custom-sa@project.iam.gserviceaccount.com"
    write_metadata        = true
  }
]

[baptistegh]

FYI @lahuang4

[lahuang4]

/gcbrun