Add support for adding/removing index aliases

Add request:

```scala
client.execute {
  aliases add "my-alias" on "my-index"
}
```

Add with all options:

```scala
client.execute {
  aliases add "my-alias" on "my-index" filter FilterBuilders.termFilter("field", "value") routing "1"
}
```

Remove request:

```scala
client.execute {
  aliases remove "my-alias" on "my-index"
}
```