[Feature] support attaching ILM policies to managed index templates

[yohancourbe]

Is your feature request related to a problem? Please describe. ILM policy is attached to an index template by setting the index.lifecycle.name on the resource elasticstack_elasticsearch_index_template which works as expected. The problem comes with managed index templates that are managed by the cluster itself (e.g. installed through Fleet integration) to which we would to attach our ILM policy, but that we wouldn't want to have managed by Terraform.

Describe the resource you would like to have implemented. We should have a resource like elasticstack_elasticsearch_index_lifecycle_index_template_attachment or something alike.

Describe the solution you'd like For managed index templates, the ILM policy should be possible to attach without managing the index template from Terraform

Describe alternatives you've considered

• Importing the managed index template and replicate its configuration - heavy workaround, not scalable
• Create the ILM with Terraform and set up the link manually - not scalable
• Create a higher priority index template that I manage - might break with integration upgrade

Additional context n/a