As mentioned above, we could support a simple alias name, or complex objects:
```
"aliases" : {
    "existingAlias",
    "simpleNewAlias",
    "newAliasWithFilter" : {
        "filter" : {
            "term" : {"user_id" : "12"}
        }
    }
}
```

This code block that parses the source needs to be adapted, maybe you can reuse `AliasMetaData#fromXContent`?