Feature Request: `enumValue.Should().BeDefined()`

Can you add an extension for this:
```cs
// Act.
var dayOfWeek = (DayOfWeek)999;

// Assert.
dayOfWeek.Should().BeOneOf(Enum.GetValues<DayOfWeek>());
Enum.IsDefined(dayOfWeek).Should().BeTrue();
// suggested: dayOfWeek.Should().BeDefined();
```


----
I use FluentAssertions Version="6.5.1".
