feat(product_enablement): implement product enablement APIs

[Integralist]

✅ I've run the end-to-end test suite on this PR and it's passing.

NOTE:
Enabling ImageOptimizer requires a backend with a 'shield' property configured.
The below example configuration demonstrates which products are available to each service type.

resource "fastly_service_vcl" "demo" {
  name = "testing-prod-enablement"

  domain {
    name    = "testing-prod-enablement.integralist.co.uk"
    comment = "demo"
  }

  backend {
    address = "127.0.0.1"
    name    = "localhost"
    port    = 80
    shield  = "amsterdam-nl"
  }

  product_enablement {
    brotli_compression = true
    domain_inspector   = true
    image_optimizer    = true
    origin_inspector   = true
    websockets         = true
  }

  force_destroy = true
}

resource "fastly_service_compute" "demo" {
  name = "testing-prod-enablement-compute"

  domain {
    name    = "testing-prod-enablement-compute.integralist.co.uk"
    comment = "demo"
  }

  package {
    filename         = "package.tar.gz"
    source_code_hash = filesha512("package.tar.gz")
  }

  product_enablement {
    fanout     = true
    websockets = true
  }

  force_destroy = true
}