[Index templates] Keep configuration of data stream when editing

[sebelga]

Fixes #87293

This PR fixes a bug in index template UI where we didn't keep the configuration of the data_stream object when editing an composable index template.

Step to reproduce

1. Create a template with a hidden configuration for the data stream

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

2. Navigate to the index template UI and edit this "test" template
3. Go to the review tab. The data_stream configuration should appear under the "Request" tab
4. Save the template
5. In console, fetching the template should still return the previous data stream config

GET _index_template/test

[sebelga]

Note that I haven't modified the "Logistics" form to be able to set the hidden parameter for the data stream. If we think it is important to support it in the UI we can do it in a separate PR (probably for the 7.12 release). WDYT @alisonelizabeth ?

[elasticmachine]

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

[alisonelizabeth]

Change LGTM. Verified locally. Thanks for fixing this @sebelga!

[alisonelizabeth]

Note that I haven't modified the "Logistics" form to be able to set the hidden parameter for the data stream. If we think it is important to support it in the UI we can do it in a separate PR (probably for the 7.12 release). WDYT @alisonelizabeth ?

👍 Per #83709, we do not need to surface in the UI:

Note: The hidden data streams feature is targeted at consumers in the Stack (elastic/elasticsearch#63889), so we won't update the Index Templates wizard with the option for defining an index template that will generate a hidden data stream.

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: a76b11e

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
indexManagement	1.5MB	1.5MB	+326.0B

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