Allow for top-level Elasticsearch mapping to be defined

[jvanderen1]

It would be nice to define an explicit Elasticsearch mapping (minus any _meta fields` at the top of the schema. Something akin to what is in the Elasticsearch documentation:

PUT /my-index-000001
{
  "mappings": {
    "properties": {
      "age":    { "type": "integer" },  
      "email":  { "type": "keyword"  }, 
      "name":   { "type": "text"  }     
    }
  }
}

[nowfred]

I believe you can do this by setting any mapping you want inside transform.mapping: {} in the schema.json file. We use this to create abstract mappings that apply to some downstream transforms - working well so far.

[jvanderen1]

@nowfred The problem is that we use custom plugins to shape the schema into a custom format before indexing. Our mappings don't necessarily align with PGSync's schema output.

We need a way to define a custom Elasticsearch mappings without referencing columns.