This feature is mainly done for the integration with the commercial reporting, but can be used
for anything else as well.
This adds a `attachments` to the email configuration, which can be used like this
```
"attachments" : {
"some_id" : {
"http" : {
"request" : {
"url" : "http://example.org/foo.pdf"
}
}
},
"other_id" : {
"data" : {
"format" : "json"
}
}
}
```
The main reason to pick this format is extensibility. If we would like to support another
attachment type, like an file reader, we could do so easily from an API point of view.
Closeselastic/elasticsearch#870
Original commit: elastic/x-pack-elasticsearch@66d14be965
75ab4f9470