[Index Patterns]: Avoid breaking saved objects when deleting an index pattern

[lukeelmers]

Currently, deleting an Index Pattern will immediately break any Saved Objects that have stored references to that Index Pattern's ID.

Now that #28199 is merged, and we are able to look up relationships for each Saved Object, we could improve the user experience by examining all Saved Objects that would be affected by the Index Pattern's removal, and doing something like:

• providing a more specific warning with a list of affected objects
• offer to delete all corresponding saved objects so that you don't have objects with stale references sitting around
• offer to rereference corresponding saved objects to another index pattern
• figure out some sort of "undefined" or orphaned state that you could put the saved object references in, so that they would be easier to identify and rereference to a new pattern later.
• create an advanced setting around one of the above, e.g. if a user wanted to prevent deleting index patterns by default if there were any objects referencing that ID