feat(alert_policy): add ability to add multiple channels to a policy

[sanderblue]

Resolves: #319

The PR introduces the ability to add alert channels to a policy during resource creation by allowing the user to supply an array of channel IDs in their Terraform config for a newrelic_alert_policy resource.

Example:

resource "newrelic_alert_policy" "foo" {
  name = "example-policy"
  channel_ids = [
    3156981,
    3103316
  ]
}

It should be noted that the user will not be able to import the channel IDs. However, a user can still import the primary resource, add channel IDs to their HCL, and then use terraform apply to add the channels to the policy.

[zlesnr]

Nice work. This looks good to me.