Convert AlertInfraCondition thresholds to pointers

[paul91]

Let me know if there is a better way to do this. The idea is that this makes it much easier to determine if these things aren't set in terraform-provider-newrelic as an optional parameter so it can be properly omitted, ie:

if attr, ok := d.GetOk("warning"); ok {
	// blahblahblah
}

If an empty warning threshold gets sent to the API it creates an invalid Alert Condition that can no longer be described without a GET resulting in a 500.

[paultyng]

I don't know if it works for the cases you are looking at, but since structs are comparable in Go:

Struct values are comparable if all their fields are comparable. Two struct values are equal if their corresponding non-blank fields are equal.

So you could just check for equality to a zero value struct. But nil is probably better, just unfortunate that its a breaking change, I'll merge and do a major version bump.

[paultyng]

Released as v2.0.0