Add json serialization routine for applies_to

[Mpdreamz]

Ensures AppliesTo is (de)serialized in a normalized fashion.

[
  {
    "type": "stack",
    "sub_type": "stack",
    "lifecycle": "ga",
    "version": "9999.9999.9999"
  }
]

This is now mapped as nested and version is mapped to version type.

"applies_to" : {
  "type" : "nested",
  "properties" : {
    "type" : { "type" : "keyword" },
    "sub-type" : { "type" : "keyword" },
    "lifecycle" : { "type" : "keyword" },
     "version" : { "type" : "version" }
  }
},

We also implement a ToString() for all of AppliesTo related types so we can cache bust the sync to semantic index if only the applies to changes.