[Index template] Data stream configuration is removed

[sebelga]

While testing #85028 I've realized that the index templates UI does not keep the information in the datastream object. There is now a new hidden parameter that can be provided.

The least we should do is forward the object as is while saving the template. I wonder if we shouln't also surface the hidden toggle.

Steps to reproduce

1. Create a template in dev tools for a hidden data stream

PUT _index_template/test
{
  "template": {},
  "index_patterns": [
    "somepattern*"
  ],
  "data_stream": {
    "hidden": true   
  }
}

2. Navigate to Index management -> Index template and edit the "test" template created
3. Without making any changes, click on the "Review template" step
4. Under the "Request" tab you can see that the hidden config has been stripped out
5. Save the template.
6. Back to console if you fetch the template the hidden config is now false

GET _index_template/test

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)