replay: add replay module, build for sitl_default, but do not load on startup

This adds a new module that does:
- read an parse an ULog file, given via ENV variable 'replay'
- apply all parameters from the log file
- read and apply user-defined override parameters from a file
- publish all messages in 'real-time' from the log file and add a constant
  offset to the timestamp to match the system time.
- apply changed parameters in the log (which are not overridden)

src/modules/logger/messages.h

@@ -49,14 +49,18 @@ enum class ULogMessageType : uint8_t {
 /* declare message data structs with byte alignment (no padding) */
 #pragma pack(push, 1)
 
-#define ULOG_MSG_HEADER_LEN 3 //accounts for msg_size and msg_type
-
 /** first bytes of the file */
 struct ulog_file_header_s {
 	uint8_t magic[8];
 	uint64_t timestamp;
 };
 
+#define ULOG_MSG_HEADER_LEN 3 //accounts for msg_size and msg_type
+struct ulog_message_header_s {
+	uint16_t msg_size;
+	uint8_t msg_type;
+};
+
 struct ulog_message_format_s {
 	uint16_t msg_size; //size of message - ULOG_MSG_HEADER_LEN
 	uint8_t msg_type = static_cast<uint8_t>(ULogMessageType::FORMAT);