AP_HAL: Add support for parity to Serial passthrough

Add code to reflect USB ACM parity setting to the passthrough port alongside existing support for ACM baud rate changes.  Some use cases for serial passthrough require specific parity settings.

For example, even parity is used and required by the USART protocol used in the STM32 system bootloader. This enhancement allows the use of standard flash programming tools such as STM32CubeProgrammer to flash connected STM based peripherals such as Receivers and Telemetry radios via serial passthrough.  Some examples of such peripherals include the FrSky R9 receivers as well as various other STM based LoRa modules used by the mLRS project.

libraries/AP_HAL_ChibiOS/UARTDriver.cpp

@@ -672,6 +672,19 @@ uint32_t UARTDriver::get_usb_baud() const
     return 0;
 }
 
+/*
+    get the requested usb parity.  Valid if get_usb_baud() returned non-zero.
+*/
+uint8_t UARTDriver::get_usb_parity() const
+{
+#if HAL_USE_SERIAL_USB
+    if (sdef.is_usb) {
+        return ::get_usb_parity(sdef.endpoint_id);
+    }
+#endif
+    return 0;
+}
+
 uint32_t UARTDriver::_available()
 {
     if (!_rx_initialised || _uart_owner_thd != chThdGetSelfX()) {
@@ -1429,6 +1442,7 @@ void UARTDriver::configure_parity(uint8_t v)
         // not possible
         return;
     }
+    UARTDriver::parity = v;
 #if HAL_USE_SERIAL == TRUE
     // stop and start to take effect
     sdStop((SerialDriver*)sdef.serial);