Add `value.IsNullOrUnknown()` function

[zliang-akamai]

Does this make sense for checking whether the value has a real value?

I have an optional but non-computed attribute, I usually think I can safely assume the plan will never be an Unknown value but am worried about someone change the schema to computed in the future and this can become a side-effect.

Writing s.IsNull() || s.IsUnknown() is verbose in provider code, IMO.

func (s StringValue) IsNullOrUnknown() bool {
	return s.IsNull() || s.IsUnknown()
}

Kind of like if someAttr, ok := d.GetOk("some_attr"); ok {} in SDKv2

[chrismarget-j]

I've often wished that the attr.Value interface included a HasValue() bool method for the same reason.

think I can safely assume the plan will never be an Unknown value

A mistake that I seem to make regularly is forgetting that while the planned value cannot be unknown for a non-Computed attribute, the config (as used exposed to attribute or resource validator) might be unknown.

[zliang-akamai]

@chrismarget-j

A mistake that I seem to make regularly is forgetting that while the planned value cannot be unknown for a non-Computed attribute, the config (as used exposed to attribute or resource validator) might be unknown.

True, I made that mistake 😭