[Derived Fields] Add parsing support for derived fields in mappings

[qreshi]

Is your feature request related to a problem? Please describe

Once the DerivedFieldMapper class is added, support for parsing it from mappings will need to be added.

Describe the solution you'd like

The definition for derived fields in the mappings will look something like the following:

"<name>": {
  "type": "keyword",
    "script": {
      "source": "<script>"
    }
}

Derived fields will be different from typical supported mapper fields in that the "type" can be any existing type (no current restrictions have been considered here yet but this issue will update if any have been identified during implementation). This would determine how the contents derived from the script are treated (ex. date, keyword, ip, etc.).

Typically from the MapperService perspective, the type is mapped to the underlying builder/parser of the Mapper classes so it's easy to identify how that field mapping would be parsed. Since type does not easily distinguish a derived field in this case though, we will add a separate named object that contains the derived fields for ease of separation in the mappings. For example:

{
  "some-index": {
    "mappings": {
      "derived": {
        "some_derived_field_name": {
          "type": "keyword",
          "script": {
            "source": "<script>"
          }
        }
      },
      "properties": {
        "some_other_field": {
          "type": keyword
        }
      }
    }
  }
}

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