Support Synthetics Tests

[gabsn]

This PR will add support for CRUD operations on API Synthetics Tests.

We need to wait for this PR to be merged before merging this one.

TODO

• support for API tests
• support for Browser tests (step creation not included)
• support for import
• update TFP doc

Testing guidelines

Either run make testacc to run acceptance tests or try the CLI tool:

# In terraform-provider-datadog
make build
cp ~/go/bin/terraform-provider-datadog ~/.terraform.d/plugins/

# In your infra repo
terraform init
terraform apply

With a basic main.tf terraform config file:

# Configure the Datadog provider
provider "datadog" {
  api_key = "XXX"
  app_key = "XXX"
}

resource "datadog_synthetics_test" "foo" {
  type = "api"

  request {
	  method = "GET"
	  url = "https://docs.datadoghq.com"
  }
  assertions = [
    {
      type = "statusCode"
      operator = "is"
      target = "200"
  	},
    {
      type = "responseTime"
      operator = "lessThan"
      target = "2000"
  	}
  ]

  locations = [ "aws:eu-central-1", "aws:ap-northeast-1" ]
  options {
	tick_every = 60
  }

  name = "name for synthetics test foo"
  message = "Notify @datadog.user"
  tags = ["foo:bar", "baz"]

  paused = true
}

[gzussa]

Let's update the API Go Client upstream.

[gzussa]

See previous comment

[bkabrda]

Overall this looks real good, I just have couple minor notes inline that should be addressed/answered. Please also consider squashing the commits. Thanks!

[bkabrda]

LGTM 👍. Code is ok, acceptance tests are green and some hand testing with TF also looks good. (I don't have rights to merge though, so someone else needs to push the button)