Plan Modifier `UseStateForUnknownIf`

[zliang-akamai]

Sometimes, the value of an attribute depends on whether another attribute's value is updated. It would be nice to have a UseStateForUnknownIf(...) function, similar to RequiresReplaceIf(...), to mark an attribute as unknown during an update operation if the attribute it depends on has been changed; otherwise, marking it as known.

func (r *MyResource) Schema(ctx context.Context, req resource.SchemaRequest, resp *resource.SchemaResponse) {
	resp.Schema = schema.Schema{
		Attributes: map[string]schema.Attribute{
			"a": schema.StringAttribute{
				Required: true,
			},
			"b_depending_on_a": schema.StringAttribute{
				Optional: true,
				PlanModifiers: []planmodifier.String{
					stringplanmodifier.UseStateForUnknownIf(
						func(ctx context.Context, sr planmodifier.StringRequest, rrifr *stringplanmodifier.UseStateForUnknownIfFuncResponse) {
							// ...
						},
					),,
				},
			},
		},
	}
}