Introduce the Ethernet transport layer

[dclause]

Description
Add support for an Ethernet-based transport layer to enable communication with remote boards/devices over wired networks.

Motivation
The current transport system lacks Ethernet support. Adding this layer will enable communication with devices connected via Ethernet, which is currently unsupported.

Requirements
Support for Arduino boards equipped with an Ethernet shield (e.g., W5100/W5500-based modules).

Notes
Ethernet transport operates over TCP, similarly to Wi-Fi. The implementation may reuse or alias existing Wi-Fi transport abstractions with minimal changes. It may also involve renaming shared components for clarity.

API Changes
None. This is an additive feature that does not affect existing APIs.