Continuous actions dsjson

[olgavrou]

Resolves #2577

continuous actions label:

It is more accurate to have pdf_value here rather than probability, so also renamed the VW::cb_continuous::continuous_label_elm probability field to pdf_value

{
  "_label_ca":
  {
    "cost": 0.657567,
    "pdf_value": 6.20426e-05,
    "action": 185.121
  },
  "feature1":1,
  "feature2":1,
  "feature2":"name1",
}