fix(webhooks): add optional service_account_id for authZ

[parkedwards]

resolves #346
resolves https://linear.app/prefect/issue/PLA-864/support-service-account-on-webhooks

in #261 , we didn't include the option to set a service_account_id, which is required if the account has Webhook Auth enabled

cc: @EmilRex

Testing

resource "prefect_service_account" "service_account" {
  name              = "service-account"
  account_role_name = "Member"
}

resource "prefect_webhook" "target_webhook" {
  name    = "webhook"
  enabled = true
  template = jsonencode({
    event = "prefect.event.received"
    resource = {
      "prefect.resource.id"   = "prefect.event"
      "prefect.resource.name" = "Prefect Event"
    }
    data = "{{ body }}"
  })

  service_account_id = prefect_service_account.service_account.id
}

# ensure that the datasource also has the correct attribute set
data "prefect_webhook" "target_webhook" {
  id = prefect_webhook.target_webhook.id
}
output "service_account_id" {
  value = data.prefect_webhook.target_webhook.service_account_id
}

result:

➜ terraform apply --auto-approve

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create
 <= read (data resources)

Terraform will perform the following actions:

  # data.prefect_webhook.target_webhook will be read during apply
  # (config refers to values not yet known)
 <= data "prefect_webhook" "target_webhook" {
      + created            = (known after apply)
      + description        = (known after apply)
      + enabled            = (known after apply)
      + id                 = (known after apply)
      + name               = (known after apply)
      + service_account_id = (known after apply)
      + slug               = (known after apply)
      + template           = (known after apply)
      + updated            = (known after apply)
    }

  # prefect_service_account.service_account will be created
  + resource "prefect_service_account" "service_account" {
      + account_id                 = (known after apply)
      + account_role_name          = "Member"
      + actor_id                   = (known after apply)
      + api_key                    = (sensitive value)
      + api_key_created            = (known after apply)
      + api_key_id                 = (known after apply)
      + api_key_name               = (known after apply)
      + created                    = (known after apply)
      + id                         = (known after apply)
      + name                       = "service-account"
      + old_key_expires_in_seconds = 0
      + updated                    = (known after apply)
    }

  # prefect_webhook.target_webhook will be created
  + resource "prefect_webhook" "target_webhook" {
      + account_id         = (known after apply)
      + created            = (known after apply)
      + enabled            = true
      + endpoint           = (known after apply)
      + id                 = (known after apply)
      + name               = "webhook"
      + service_account_id = (known after apply)
      + template           = jsonencode(
            {
              + data     = "{{ body }}"
              + event    = "prefect.event.received"
              + resource = {
                  + "prefect.resource.id"   = "prefect.event"
                  + "prefect.resource.name" = "Prefect Event"
                }
            }
        )
      + updated            = (known after apply)
      + workspace_id       = (known after apply)
        # (1 unchanged attribute hidden)
    }

Plan: 2 to add, 0 to change, 0 to destroy.

Changes to Outputs:
  + service_account_id = (known after apply)
prefect_service_account.service_account: Creating...
prefect_service_account.service_account: Creation complete after 0s [id=6bf0eada-f6f0-4e9a-ac60-c29ba137c85c]
prefect_webhook.target_webhook: Creating...
prefect_webhook.target_webhook: Creation complete after 1s [id=7b9f4f3d-4c87-489b-b773-5f84aa7c1915]
data.prefect_webhook.target_webhook: Reading...
data.prefect_webhook.target_webhook: Read complete after 0s [id=7b9f4f3d-4c87-489b-b773-5f84aa7c1915]

Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

Outputs:

service_account_id = "6bf0eada-f6f0-4e9a-ac60-c29ba137c85c"