Add support for discriminator identification for anyOf

[jdesrosiers]

I figured out a way to identify discriminators by adding nodes in the errorIndex for properties that don't exist in the instance. There's still a lot more to figure out, but this gets us passed one hurdle.

[arpitkuriyal]

Looks good. Sorry for the delay i got busy with some personal work. I am looking to the all TODO still figuring out how to handle all this case.
Case 1: Handle alternatives with const - In this we can check the type of the const value and generate error message accordingly

 {
  "anyOf": [
    { "const": "READ_ONLY" },
    { "const": 4 }
  ]
}

Like in this if instance: 5 we can simply check the type but what if both alternative type is also same (then only option i can see is to throw error message for each alternative separately)

Each TODOs gets more complex when i think more and more about how to handle it 🙂