Add `value.IsNullOrUnknown()` function

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.

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

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