http-specs, add test case for ArrayEncoding with enum

Add a new test case `op pipeDelimitedEnumElement(@body body: PipeDelimitedArrayEnumElementProperty): PipeDelimitedArrayEnumElementProperty;)` to https://github.com/microsoft/typespec/blob/main/packages/http-specs/specs/encode/array/main.tsp

The scenario on wire would be same as `pipeDelimited`.

Just the type be e.g.

```
model PipeDelimitedArrayEnumElementProperty {
@encode(ArrayEncoding.pipeDelimited)
value: Color[];
}
```