Field API path syntax support in IngestDocument

[felixbarny]

A POC for allowing a field api syntax for ingest pipeline field references.

This doesn't require any changes to WriteField or the field api itself. It just exposes the existing functionality in the context of ingest pipeline field references.

Example:

{
  "processors": [
    {
      "rename": {
        "field": "$('attributes.foo.bar')",
        "target_field": "$('attributes.bar.foo')"
      }
    }
  ]
}