This adds an `include_type_name` option to the `indices.create`,
`indices.get_mapping` and `indices.put_mapping` APIs, which defaults to `true`.
When set to `false`, then mappings will be returned directly in the body of
the `indices.get_mapping` API, without keying them by the type name, the
`indices.create` will expect mappings directly under the `mappings` key, and
the `indices.put_mapping` will use `_doc` as a type name and fail if a `type`
is provided explicitly.

Relates elastic#15613