validate filter length doesn't exceed 256 bytes in subscription

[modular-magician]

This PR validates that the filter length doesn't exceed 256 bytes in a PubSub subscription, on TF it should fail when planning

Release Note Template for Downstream PRs (will be copied)

pusbub: added validation to `filter` field for `google_pubsub_subscription`

Derived from GoogleCloudPlatform/magic-modules#11421

[philvmx1]

@modular-magician this change caused our builds to fail since we have 0-length subscriptions (empty string).

Please reference the API documentation for "filter" https://cloud.google.com/pubsub/docs/reference/rest/v1/projects.subscriptions/create

It implicitly says that empty strings are allowed.

[ndopj]

@modular-magician this is breaking change which was not released under new Major version. The API allows filters to contain new line characters. This is useful when Terraform users use Heredoc strings for attributes expecting string type.

Example for PubSub filter:

resource "google_pubsub_subscription" "events" {
  name   = "events"
  topic  = google_pubsub_topic.events.name
  filter = <<-EOT
  (attributes.platform = "web" AND attributes.category = "shopping_cart")
  OR (attributes.platform = "app")
  EOT
}

The above filter used to work before the change introduced in this MR. Introduced regex, ^.{1,256}$, used for validation of the filter attribute does not allow new lines to be present in the provided string value.

[github-actions]

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