Add option for http_cache_hosts

To enable Varnish in env.php an additional command needs to be executed:
`bin/magento setup:config:set --http-cache-hosts=varnish`

It seems there is no possibility to append

```
'http_cache_hosts' => [
[
'host' => 'varnish'
]
]
```

to env.php in ece-tools.

Start by tracing how ece-tools generates env.php and handles the setup:config:set --http-cache-hosts=varnish command. Check how the requested http_cache_hosts structure is represented and whether existing configuration generation coverage exercises it. Done means ece-tools can produce the equivalent env.php configuration without requiring the additional command.

Written by the indexing model from the issue text.