Add powsybl-optimizer used by study-server and voltage-init-server

Author: jonenst

.gitmodules

@@ -290,3 +290,7 @@
 	path = backend/build/powsybl-ws-dependencies
 	url = https://github.com/powsybl/powsybl-ws-dependencies
 	branch = main
+[submodule "backend/libs/powsybl-optimizer"]
+	path = backend/libs/powsybl-optimizer
+	url = https://github.com/powsybl/powsybl-optimizer
+	branch = main

backend/libs/pom.xml

@@ -30,6 +30,7 @@
         <module>powsybl-network-hypothesis</module>
         <module>powsybl-network-store</module>
         <module>powsybl-open-loadflow</module>
+        <module>powsybl-optimizer</module>
         <module>powsybl-ws-commons</module>
     </modules>