Enum list options not displayed in help text

Apologies if this question/bug was already open

**Describe the bug**
The property: `HelpText.AddenumValuesToHelpText` does not work when the option is of type `IEnummerable<{Enum}>`. I'm guessing the list is masking the fact that its containing an enum.

Is it possible to have the same `Valid values: {enum values}` in the help text to be displayed when its a list of enums?

**To Reproduce**
1. Create an option of type `IEnummerable<{Enum}>`
2. configure a parser as follows:
```cs
var parser = new Parser(with =>
{
with.CaseInsensitiveEnumValues = true;
with.HelpWriter = null;
with.AutoVersion = false;
});

var parserResult = parser.ParseArguments(args.SplitArgs());
parserResult
.WithNotParsed(_ =>
{
var helpText = HelpText.AutoBuild(parserResult, h =>
{
h.AutoVersion = false;
h.Heading = string.Empty;
h.Copyright = string.Empty;
h.AddEnumValuesToHelpText = true;

return h;
});
Console.WriteLine(helpText);
});
```
3. use the --help to find no `Valid values: {enum values}` in the help text.

**Expected behavior**
I expected a list of enums to still show available options

**Screenshots**

首先，使用报告中所示的 Parser 和 HelpText.AutoBuild 配置，复现类型为 IEnumerable 的选项所出现的问题。跟踪 AddEnumValuesToHelpText 如何处理集合选项类型；完成的标准是，生成的 --help 输出包含列表选项的 enum 有效值，同时保留标量 enum 的现有行为。

由索引模型根据 Issue 内容生成。