Add fields to site integrations

[jeremy-cxf]

Currently, you cannot add datadog integrations via the integrations resource.

This adds the fields parameters required (fields) to be expressed as it is in main.tf

resource "sigsci_site_integration" "test_integration_datadog" {
  site_short_name = sigsci_site.my-site.short_name
  type            = "datadog"
  url             = ""
  events          = ["flag", "agentAlert"]

  fields{
      name  = "site"
      value = "us1"
  }

  fields{
      name  = "apiKey"
      value = "your_api_key"
  }
}

This is reliant on updating go-sigsci: https://github.com/signalsciences/go-sigsci/pull/65/files so pipeline builds will fail.

I would much rather get testing setup for these though, but it appears slightly broken? Will raise in the appropriate channels.