Improve W5500 Ethernet configuration and diagnostics

[BanjoR]

Summary

• add platform-gated W5100/W5500 CS selection via ETHERNET_CS and ETHERNET_CS_PIN
• add an ESP32-Ethernet PlatformIO environment with configurable SPI and reset pins, while disabling Wi-Fi for wired builds
• add hardware, link, DHCP, hostname, and IP diagnostics
• document DHCP behavior, SSID irrelevance for wired Ethernet, wiring, and a repeatable Mega/ESP32 W5500 validation procedure

Scope and context

This refresh starts from current upstream master and consolidates the relevant portions of stale PRs #316, #317, and the wrong-base ESP32 draft #525. It covers the configuration and diagnostic portions of the open reports #308, #313, and #375.

The official Arduino Ethernet dependency requests DHCP automatically when IP_ADDRESS is not defined and currently hard-codes its DHCP hostname option. ETHERNET_HOSTNAME is therefore applied to DCC-EX mDNS and native STM32 Ethernet, and is reported at startup; changing the W5500 DHCP hostname in DCC-EX#313 requires an upstream dependency API/library update rather than a safe CommandStation-only patch. See the dependency source: https://github.com/arduino-libraries/Ethernet/blob/master/src/Dhcp.cpp

Validation

• git diff --check passed
• static allowlist and platform-environment assertions passed
• PlatformIO firmware builds could not run here because PlatformIO/C++ toolchains are not supplied
• Mega/ESP32 W5500 hardware validation remains pending and is documented in config.example.h

No source changes from the ESP32 OTA portion of DCC-EX#525 are included.

[BanjoR]

Superseded by upstream-targeted PR DCC-EX#541.