[Fleet] Editing ILM policies is really difficult since they get overwritten each package update

[andrewvc]

There's no great way to change retention or other ILM settings with fleet. We do document how to do it with custom component templates, but the process is embarassingly painful, especially for the common task of updating an existing ILM policy's lifecycle only.

Ideally users would be able to:

1. Go to a data stream in kibana stack management
2. Click on the datastream
3. Click on the ILM policy
4. Customize the policy and save it

and have it persist.

This actually does work today, but the issue is that on the next package update the change will be overwritten.

I'm not certain how to best solve this, but ideally we'd just have one lifecycle policy that is updated unless a user has modified it.

CC @ruflin who requested I make this issue after a discussion about UX here

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[wurstbrotfan]

from an Elastic user's persepective I agree that the recommended way of editing custom component templates in order to change lifecycle policy is "painful". Some integrations come with multiple data streams, thus for a new system multiple custom component templates need to be edited.

[ipaqmaster]

It would be nice if we could attach our custom/existing non-stack-managed policies to Fleet policies instead of watching cluster's with Fleet/Elastic Agent's fail to manage the storage correctly.