Our API spec define the tasks API as e.g. tasks.list, meaning that they belong to their own namespace. This commit moves them from the cluster namespace to their own namespace.
Relates to #29546
This change adds a `listTasks` method to the high level java
ClusterClient which allows listing running tasks through the
task management API.
Related to #27205
As part of adding support for new API to the high-level REST client,
we added support for the `flat_settings` parameter to some of our
request classes. We added documentation that such flag is only ever
read by the high-level REST client, but the truth is that it doesn't
do anything given that settings are always parsed back into a `Settings`
object, no matter whether they are returned in a flat format or not.
It was a mistake to add support for this flag in the context of the
high-level REST client, hence this commit removes it.
