http-specs, add test case for ArrayEncoding with enum

[weidongxu-microsoft]

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[];
}