Refactor RestClientTransport to allow using other http client libraries

[swallez]

Extract the logic related to high-level API objects (serialization, deserialization, error handling, etc) that exists in RestClientTransport to a new abstract ElasticsearchTransportBase class. This class delegates http processing to implementations of a new TransportHttpClient interface.

The goal is to allow additional implementations to be provided, such as one based on the builtin http client in JDK11+, and allow users to easily implement their own when needed. The new example-transports sub-project contains an example implementation that uses Netty.

Note that ElasticsearchTransportBase does not implement multi-node retry and cluster sniffing, which is provided by RestClient and would have to be re-implemented for other http libraries.

Fixes #550