This PR adds support for the Get Settings API to the java high-level rest client.
Furthermore, logic related to the retrieval of default settings has been moved from the rest layer into the transport layer and now default settings may be retrieved consistency via both the rest API and the transport API.
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.
Nhat Nguyen
tomcallahan
Luca Cavanna
olcbean
Mayya Sharipova
Yu