Added freeopcua library

.gitmodules

 [submodule "libs/spdlog"]
 	path = libs/spdlog
 	url = https://github.com/gabime/spdlog
+
+[submodule "libs/freeopcua"]
+	path = libs/freeopcua
+	url = https://github.com/FreeOpcUa/freeopcua
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