serial: add txSpaceAvailable function (#26069)

* serial: add txSpaceAvailable function

* serial: txSpaceAvailable and bytesAvailable fixups

platforms/common/Serial.cpp

@@ -74,6 +74,11 @@ ssize_t Serial::bytesAvailable()
 	return _impl.bytesAvailable();
 }
 
+ssize_t Serial::txSpaceAvailable()
+{
+	return _impl.txSpaceAvailable();
+}
+
 ssize_t Serial::read(uint8_t *buffer, size_t buffer_size)
 {
 	return _impl.read(buffer, buffer_size);

platforms/common/include/px4_platform_common/Serial.hpp

@@ -62,6 +62,7 @@ public:
 	bool close();
 
 	ssize_t bytesAvailable();
+	ssize_t txSpaceAvailable();
 	ssize_t read(uint8_t *buffer, size_t buffer_size);
 	ssize_t readAtLeast(uint8_t *buffer, size_t buffer_size, size_t character_count = 1, uint32_t timeout_ms = 0);