Add hidden match property handling

[mkatychev]

Store the selector string of any found secret variables so that a sensible diff can be created without leaking secrets.

Description
When request/response mismatch occurs, hidden values (ex. _HIDDEN_VAL) are present in output when not relevant to mismatch.
Steps to Reproduce:

• Trigger a diff that will still show the leaked value in the actual response:

> _HIDDEN
< leaked_value
Expected Result(s)
> _HIDDEN_SHA_VALUE
< _HIDDEN_SHA_VALUE

Stripe API example:

{
  "cut": {
    "from": [
      "STRIPE_API",
      "_STRIPE_KEY"
    ],
    "to": {
      "STRIPE_TOKEN": "'response'.'body'.'id'"
    }
  },
  "protocol": "HTTP",
  "request": {
    "header": {
      "Authorization": "Bearer ${_STRIPE_KEY}"
    },
    "uri": "POST /v1/tokens",
    "query": {
      "card[number]": 4242424242424242,
      "card[exp_month]": 4,
      "card[exp_year]": 2025,
      "card[cvc]": 314
    },
    "entrypoint": "${STRIPE_API}"
  },
  "response": {
    "validation": {
      "'response'.'body'": {
        "partial": true
      }
    },
    "body": {
      "id": "${_STRIPE_TOKEN}",
       // mismatch fails on a non-hidden value but _STRIPE_TOKEN value will be included in diff
      "something": "else"
    },
    "status": 200
  }
}

Produces this error:

< left / > right
 {
   "body": {
<    "id": "${_STRIPE_TOKEN}",
<    "something": "else"
>    "id": "tok_1J0B3Z2eZvKYlo2CjNjCNDG2"
   },
   "status": 200
 }