Add support for join field mappings

[csh97]

Add support for join field types in index mappings

See Elasticsearch documentation: https://www.elastic.co/guide/en/elasticsearch/reference/current/parent-join.html

The documentation has the following example of a simple join field:

PUT my-index-000001
{
  "mappings": {
    "properties": {
      "my_id": {
        "type": "keyword"
      },
      "my_join_field": { 
        "type": "join",
        "relations": {
          "question": "answer" 
        }
      }
    }
  }
}

With the added JoinDefinition and join() you can now replicate this:

IndexSettingsAndMappingsDSL().apply {
        mappings {
            join("my_join_field") {
                relations("question" to listOf("answer"))
            }
        }
    }.json(pretty = true).let(::println)

This produces:

{
  "mappings": {
    "properties": {
      "my_join_field": {
        "type": "join",
        "relations": {
          "question": [
            "answer"
          ]
        }
      }
    }
  }
}

The only difference here is question is always a list even if there is only 1 value, as far as I'm aware this shouldn't be a problem

The documentation also has a slightly more complex example with multiple levels of relation:

PUT my-index-000001
{
  "mappings": {
    "properties": {
      "my_join_field": {
        "type": "join",
        "relations": {
          "question": ["answer", "comment"],  
          "answer": "vote" 
        }
      }
    }
  }
}

This can also be replicated like so:

 IndexSettingsAndMappingsDSL().apply {
        mappings {
            join("my_join_field") {
                relations("question" to listOf("answer", "comment"), "answer" to listOf("vote"))
            }
        }
    }.json(pretty = true).let(::println)

Which produces:

{
  "mappings": {
    "properties": {
      "my_join_field": {
        "type": "join",
        "relations": {
          "question": [
            "answer", 
            "comment"
          ],
          "answer": [
            "vote"
          ]
        }
      }
    }
  }
}

[jillesvangurp]

Awesome. I think you would also need the related queries and aggrgegation support to use this. But that can be done later with another PR if you feel up to it.

[csh97]

My current use case is just for create indices and insert data, so I dont need the queries right now. However, you are right it would be great if the library supported it. I'm happy to have a look at adding support for this though

[jillesvangurp]

Thanks for this. I'll merge this.