SetNew does not work on nested fields

### SDK version
```
{
"Path": "github.com/hashicorp/terraform-plugin-sdk",
"Version": "v1.12.0"
}
```

### Use-cases
Currently, trying to set any nested (computed) field value in a `CustomizeDiff` function will fail due to a hardcoded `false` value on `d.checkKey()` in [`schema.ResourceDiff.SetNewComputed()`](https://github.com/hashicorp/terraform-plugin-sdk/blob/master/helper/schema/resource_diff.go#302) and [`schema.ResourceDiff.SetNew()`](https://github.com/hashicorp/terraform-plugin-sdk/blob/master/helper/schema/resource_diff.go#L290).

This seems to have been changed in commit [1e08e98](https://github.com/hashicorp/terraform-plugin-sdk/commit/1e08e982731eb0c0a35fc00dcc3878bb1f790f70#diff-5572654f34bded06e0d3e5eb9ed7d1bf). Not entirely sure why this change was made, and in any case, the Elasticsearch property is a list, but not a computed one, so not even that would be possible to Get and Set.

My use case is having global values in resources which are used to override nested values.

```go
func Deployment() *schema.Resource {
return &schema.Resource{
Schema: map[string]*schema.Schema{
"version": {
Type: schema.TypeString,
Required: true,
},

// Workloads
"elasticsearch": {
Type: schema.TypeList,
MinItems: 1,
MaxItems: 1,
Required: true,
Elem: &schema.Resource{
Schema: map[string]*schema.Schema{
// ...
"version": {
Type: schema.TypeString,
Computed: true,
},
},
},
},
// ...
},
CustomizeDiff: customdiff.All(
customdiff.IfValueChange("version", func(old, new, meta interface{}) bool {
return new.(string) > old.(string)
}, func(d *schema.ResourceDiff, _ interface{}) error {
// Succeeds obtaining the key.
log.Println("[DEBUG]", d.Get("elasticsearch.0.version"))
// Fails setting ANY nested key.
if err := d.SetNew("elasticsearch.0.version", d.Get("elasticsearch.0.version")); err != nil {
return err
}
return nil
}),
),
}
}
```

### Proposal

Make `SetNewComputed` and `SetNew` able to set items on a list.

Start in helper/schema/resource_diff.go at ResourceDiff.SetNewComputed(), ResourceDiff.SetNew(), and the d.checkKey() calls referenced by the issue. Review commit 1e08e98 and the nested list example, then verify that setting a nested list item succeeds without breaking existing key validation; the issue is done when both methods support the described nested use case.

Written by the indexing model from the issue text.