Introduce elasticsearch nio library jar

[Tim-Brooks]

Currently our nio work lives in :test:framwork. This allows us to use nio as the basis for a test TcpTransport implementation in internal cluser integration tests. This verification is very valuable and we do not want to lose it. However, we need the base nio facilities to be available for creating a production TcpTransport implementation (in a plugin/module) and a production http transport implementation. This means that the base nio facilities need to be available elsewhere (while remaining available in :test:framwork).

In order to do this, we need to introduce a nio library jar that provides the building blocks for creating transports based on nio. Ideally, this jar will not depend on elasticsearch:core as we would like to keep open the possibility that this nio work could eventually drive the rest client.

This is related to #27260.

[Tim-Brooks]

Closed by 06b3130.