By making RestHighLevelClient Closeable, its close method will close the internal low-level REST client instance by default, which simplifies the way most users interact with the high-level client.
Its constructor accepts now a RestClientBuilder, which clarifies that the low-level REST client is internally created and managed.
It is still possible to provide an already built `RestClient` instance, but that can only be done by subclassing `RestHighLevelClient` and calling the protected constructor that accepts a `RestClient`. In such case a consumer has also to be provided, which controls what has to be done when the high-level client gets done.
Closes#26086
6d8e2c6d4c