[protocols] Add CoAP support

cmake/OssiaDeps.cmake

@@ -103,6 +103,13 @@ include(deps/unordered_dense)
 include(deps/verdigris)
 include(deps/websocketpp)
 
+if(OSSIA_PROTOCOL_COAP)
+  include(deps/coap)
+  if(NOT TARGET libcoap::coap-3)
+    set(OSSIA_PROTOCOL_COAP FALSE CACHE INTERNAL "" FORCE)
+  endif()
+endif()
+
 if(OSSIA_PROTOCOL_MQTT5)
   include(deps/mqtt)
   if(NOT TARGET Async::MQTT5)

cmake/deps/coap.cmake

@@ -0,0 +1,22 @@
+if(OSSIA_USE_SYSTEM_LIBRARIES)
+    find_path(LIBCOAP_INCLUDEDIR coap3/libcoap.h)
+    find_library(LIBCOAP_LIBRARIES coap-3)
+    add_library(coap-3 IMPORTED SHARED GLOBAL)
+    add_library(libcoap::coap-3 ALIAS coap-3)
+    target_include_directories(coap-3 INTERFACE "${LIBCOAP_INCLUDEDIR}")
+    set_target_properties(coap-3 PROPERTIES IMPORTED_LOCATION "${LIBCOAP_LIBRARIES}")
+endif()
+
+if(NOT TARGET libcoap::coap-3)
+    include(FetchContent)
+    FetchContent_Declare(
+      libcoap
+      GIT_REPOSITORY "https://github.com/obgm/libcoap"
+      GIT_TAG develop
+      GIT_PROGRESS true
+    )
+
+    set(ENABLE_DTLS OFF)
+    FetchContent_MakeAvailable(libcoap)
+endif()
+