[ML] Allow user settings for data frame analytics destination index

[dimitris-athanasiou]

There are 2 scenarios wrt to the destination index of a data frame analytics job:

1. The index is not created when the job starts; then the job creates the index and copies over source indices mappings
2. The index has been already created when the job starts; then the job assumes the user customized index settings and mappings and only adds the analysis result object mappings.

However, there is a use case where the user does not want to deal with mappings copying, yet they need to specify extra settings (e.g. change fields limit, etc.). We can address this by adding the ability to specify index settings in the dest index config of a data frame analytics job.

[elasticmachine]

Pinging @elastic/ml-core (:ml)

[przemekwitek]

Just a random idea: could this be handled by the user providing the index template (containing settings) instead?
Or do you think having this settings section in DFA config would simplify overall UX significantly?

[dimitris-athanasiou]

Note that there are now plans for an API that allows preparing a target index for reindex #68857. I think this might solve all of our concerns here.

[dimitris-athanasiou]

As of #91546 we now copy more settings from the source indices to the destination index.