[datadog_synthetics_test] Fix http2 pseudo-headers not accepted

[Drarig29]

After https://github.com/DataDog/terraform-provider-datadog/pull/2469, the following Terraform definition:

resource "datadog_synthetics_test" "test" {
  [...]
  request_headers = {
    ":authority" = "example.com"
  }
  [...]
}

Is failing with:

│ Error: invalid value for request_headers (header field must be a valid token)
│
│ with datadog_synthetics_test.test,
│ on modules/uptime-monitoring/main.tf line 51, in resource "datadog_synthetics_test" "test":
│ 51: request_headers = {
│ 52: ":authority" = "example.com"
│ 54: }