Add multi_field support for Mapper externalValue (plugins)

In context of mapper attachment and other mapper plugins, when dealing with multi fields, sub fields never get the `externalValue` although it was set.

Here is a full script which reproduce the issue when used with mapper attachment plugin:

```
DELETE /test

PUT /test
{
    "mappings": {
        "test": {
            "properties": {
                "f": {
                    "type": "attachment",
                    "fields": {
                        "f": {
                            "analyzer": "english",
                            "fields": {
                                "no_stemming": {
                                    "type": "string",
                                    "store": "yes",
                                    "analyzer": "standard"
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

PUT /test/test/1
{
    "f": "VGhlIHF1aWNrIGJyb3duIGZveGVz"
}

GET /test/_search
{
    "query": {
        "match": {
           "f": "quick"
        }
    }
}

GET /test/_search
{
    "query": {
        "match": {
           "f.no_stemming": "quick"
        }
    }
}

GET /test/test/1?fields=f.no_stemming
```

Related to https://github.com/elasticsearch/elasticsearch-mapper-attachments/issues/57

Closes #5402.