Add HAL Driver for EX-IOExpander connectivity over TCP/IP (Proof of Concept)

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Adds a TCP client transport for EX-IOExpander's, so CommandStation can talk to one or more IOExpander's over a network stack (WiFi/Ethernet) instead of I2C.

This implementation is very preliminary and not well tested. I'm sharing as a proof of concept rather than a complete implementation.

This is the CommandStation side of the codebase, for the IOExpander, please see:
DCC-EX/EX-IOExpander#65

This code builds off another PR which introduced shift register support for IOExpander:
#501

• Introduces a TCP-based EX-IOExpander HAL implementation that mirrors the current request/response behavior used over I2C.
• Supports multiple IOExpanders concurrently by instantiating one TCP connection per configured expander (unique IP per device, shared port supported).
• Keeps the existing EX-IOExpander command set intact (INIT / pin map / read digital+analog / write digital+analog / shift-in/out / version), but frames messages for TCP.
• Each "EXIOExpander_TCP" entry connects to <expander_ip>:<port> and performs EXIOINIT to validate pin-count and set first vPIN and subsequent poll cycles for EXIORDD / EXIORDAN and on-demand writes (EXIOWRD, EXIOWRAN, etc.)
• Uses a simple frame format: [CMD][LEN][PAYLOAD...] with corresponding responses.
• Error handling includes timeouts and reconnect on disconnect.

The HAL driver can be called using the following:

HAL(EXIOExpander_TCP, VPIN, NUM_PINS, IP ADDRESS, OPTIONAL_PORT )

Examples:
HAL(EXIOExpander_TCP, 800, 16, 192, 168, 1, 200) // default port 2560
HAL(EXIOExpander_TCP, 800, 16, 192, 168, 1, 200, 2560) // explicit port