Additions to the Serial UART API (#22953)

- Added an empty constructor, setPort, and validatePort functions for Serial API
- Changed GPS to not allocate Serial object dynamically
- Moved access check on serial port name into the Serial API
- Improved the Qurt platform validatePort Serial function to implement a more rigorous check. Added safety check
to the setPort Serial function to make sure it isn't called after the port has been already opened.

platforms/common/include/px4_platform_common/Serial.hpp

@@ -48,6 +48,7 @@ namespace device __EXPORT
 class Serial
 {
 public:
+	Serial();
 	Serial(const char *port, uint32_t baudrate = 57600,
 	       ByteSize bytesize = ByteSize::EightBits, Parity parity = Parity::None,
 	       StopBits stopbits = StopBits::One, FlowControl flowcontrol = FlowControl::Disabled);
@@ -83,6 +84,8 @@ public:
 	FlowControl getFlowcontrol() const;
 	bool setFlowcontrol(FlowControl flowcontrol);
 
+	static bool validatePort(const char *port);
+	bool setPort(const char *port);
 	const char *getPort() const;
 
 private: