Bridge: add document about bridge header protocal

docs/specs/bridge_header_protocal.md

+# Bridge Header Protocal
+This document provides the details about bridge header protocal.
+
+## Introduction
+Bridge is responsible for forwarding the proto message specified in Apollo to the external module in UDP mode, and receiving the proto message from the external module.
+Due to the limitation of the UDP packet (the maximum packet size is 65535 bytes), the proto message exceeds this limit after serialization.
+It is possible, so the serialized packets need to be fragmented, and each slice is called a frame.
+The receiver receives all the frames and then combines and deserializes them.
+In order to conveniently distinguish each frame of data, to facilitate subsequent combination and deserialization,
+it is necessary to add frame header information at the head of each frame of data, as shown in the following figure:
+    +-----+------------+------------+-------+-----------+
+    |Flag |Header Size |Header Item |... ...|Header Item|
+    +-----+------------+------------+-------+-----------+
+As shown in the figure above, the Header divides three segments: Flag, Size, and Items, where the segment is separated by '\n'.
+Flag: The sign of this agreement, currently a string "ApolloBridgeHeader";
+Size: the length of the entire Items section (unit: byte)
+Items: The specific content of the header, where each item has three components, each of which is divided by ':'. The specific schematic is shown in the figure below.
+    +----------+----------+-------------+
+    |Item Type |Item Size | Item Content|
+    +----------+----------+-------------+
+The Item Type is defined as follows:
+Enum HType {
+Header_Ver, // header version number, type: uint32_t
+Msg_Name,   //proto name, such as Chassis, type: std::string
+Msg_ID,     // sequence num of proto, guaranteed unique value in the same proto, type: uint32_t
+Msg_Size,   // the size of the proto serialized, type: size_t
+Msg_Frames, // how many frames are proto serialized, type: uint32_t
+Frame_Size, // the size of the current frame, type: size_t
+Frame_Pos   // The offset of the current frame in the meta-serialized buf, type: size_t
+Frame_Index,// The sequence number of the current frame in the total number of frames, starting from 0, type: uint32_t
+Time_Stamp, // timestamp, type: double
+Header_Tail,// indicates the maximum value of the currently supported Type, always at the end of the enum
+}