Add serial parity option (M575 F) for device/Modbus mode

[filippo-bosi]

Implements #1196 (parity bit for RS485 Modbus).

What's changed

Aux serial ports in device (Modbus RTU) mode previously always opened as 8N1. Some Modbus devices require parity — e.g. OnRobot grippers need 8E1 — so this adds an optional parity setting to M575:

• AuxDevice: stores a SerialParity (none/even/odd) and applies it in SetMode() by selecting Mode_8N1/8E1/8O1 when calling AsyncSerial::begin(baudRate, config).
• M575: in device mode, parses a new F parameter — F0 none/8N1 (default), F1 even/8E1, F2 odd/8O1. The parameter is applied on every device-mode M575, so omitting F gives 8N1 exactly as before this option existed, rather than retaining a previously set parity.

Example: M575 P1 S7 B115200 F1 → Modbus RTU, 115200 baud, 8E1.

Notes

• No core change needed: CoreN2G's AsyncSerial already defines the Mode_8E1/8O1 framing constants.
• Default stays 8N1, so PanelDue/raw modes are unaffected.
• F doesn't clash with any existing M575 parameter.
• Parity is bus-wide (as suggested in the issue); per-slave parity via M260/M261 is not included in this PR.
• I've been running this in production on 3.6.2-based builds integrating an OnRobot gripper (8E1); this PR is the same change rebased onto 3.7-dev.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[filippo-bosi]

I have read the Duet3D CLA v2.0 and I hereby sign it

[dc42]

Unfortunately this change broke the Duet 3 Mini build. To fix it I have had to refactor the change to move the hardware detail into CoreN2G. @filippo-bosi please integrate these changes into your fork and test that you are still able to use even parity on Modbus.