Add basic support for duinoNodes shift register handling via IOExpander

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I have been tinkering with a personal fork of the EX-IOExpander codebase to see if I could add shift-register support.

My primary objective was to add support for duinoNodes (DNIN8, DNOU8, etc.) directly connected to an Arduino running EX-IOExpander.

This is the CommandStation-EX portion of the codebase, please use it for reference as needed. For the EX-IOExpander code, please visit this PR:
DCC-EX/EX-IOExpander#64

Adds a new IODevice implementation that exposes DuinoNodes-style shift-register nodes via an EX-IOExpander device.

1. Introduces IO_DNShiftExpander, an IODevice subclass that bridges VPIN-based configuration (HAL config) <-> EX-IOExpander shift commands (I2C).

2. The device is configured with:

• A VPIN range for the logical DN node (e.g. 201–232).
• 3x EX-IOExpander-backed VPINs for clk, latch & data.
• Input vs Output mode, plus optional per-bit remapping (pinMap) to match DNIN8/DNOU8 board bit ordering.

3. At runtime:

• Input mode: Periodically polls the expander using shiftInBytes() and updates the local bitmap. _read() serves per-VPIN state from that bitmap.
• Output mode: _write() updates the bitmap and marks transmit pending. _loop() flushes via shiftOutBytes() when needed.

4. Includes some basic checks to ensure the 3x control VPINs all belong to the same EX-IOExpander instance and are within its VPIN range.

5. Adds a _display() implementation so the device is identified in diagnostics as a DNIN/DNOU node “via EXIO” instead of showing up as an unknown device.

To simplify the implementation, I created new device classes called IO_DNIN8V, IO_DNIN8KV & IO_DNOU8V which can be used to reference a duinoNode on an IOExpander. For example:

HAL(EXIOExpander, 401, 16, 0x65)
HAL(IO_DNIN8V, 201, 32, 409, 410, 411)

The above example defines an EX-IOExpander and assigned a DNIN8 duinoNode at VPIN 409, 410 & 411 on that Expander.