Add version macros

Add version.h with the following values. This values are managed/updated
via CMake

```
#pragma once

#define RTC_VERSION_MAJOR 0
#define RTC_VERSION_MINOR 20
#define RTC_VERSION_PATCH 1
#define RTC_VERSION "0.20.1"
```

# Please enter the commit message for your changes. Lines starting
# with '#' will be kept; you may remove them yourself if you want to.
# An empty message aborts the commit.
#
# Date:      Fri Feb 16 10:25:47 2024 -0500
#
# On branch version-macro
# Changes to be committed:
#	modified:   CMakeLists.txt
#	new file:   cmake/version.h.in
#	modified:   include/rtc/rtc.h
#	new file:   include/rtc/version.h
#
# Untracked files:
#	install/
#

CMakeLists.txt

@@ -118,6 +118,7 @@ set(LIBDATACHANNEL_HEADERS
 	${CMAKE_CURRENT_SOURCE_DIR}/include/rtc/rtcpnackresponder.hpp
 	${CMAKE_CURRENT_SOURCE_DIR}/include/rtc/utils.hpp
 	${CMAKE_CURRENT_SOURCE_DIR}/include/rtc/plihandler.hpp
+	${PROJECT_BINARY_DIR}/include/rtc/version.h
 )
 
 set(LIBDATACHANNEL_IMPL_SOURCES
@@ -252,6 +253,11 @@ else()
 	add_library(Usrsctp::Usrsctp ALIAS usrsctp)
 endif()
 
+configure_file (
+    ${PROJECT_SOURCE_DIR}/cmake/version.h.in
+	${CMAKE_INSTALL_INCLUDEDIR}/rtc/version.h
+)
+
 add_library(datachannel SHARED
 	${LIBDATACHANNEL_SOURCES}
 	${LIBDATACHANNEL_HEADERS}

cmake/version.h.in

@@ -0,0 +1,6 @@
+#pragma once
+
+#define RTC_VERSION_MAJOR @PROJECT_VERSION_MAJOR@
+#define RTC_VERSION_MINOR @PROJECT_VERSION_MINOR@
+#define RTC_VERSION_PATCH @PROJECT_VERSION_PATCH@
+#define RTC_VERSION "@PROJECT_VERSION@"

include/rtc/rtc.h

@@ -9,6 +9,8 @@
 #ifndef RTC_C_API
 #define RTC_C_API
 
+#include "version.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif

include/rtc/version.h

@@ -0,0 +1 @@
+