chore: add reactor-core to core lib deps

PiperOrigin-RevId: 794330264

Author: Poggecci

core/pom.xml

@@ -178,6 +178,11 @@
       <artifactId>rxjava</artifactId>
       <version>3.1.5</version>
     </dependency>
+    <dependency>
+      <groupId>io.projectreactor</groupId>
+      <artifactId>reactor-core</artifactId>
+      <version>3.7.0</version>
+    </dependency>
     <dependency>
       <groupId>com.github.tomakehurst</groupId>
       <artifactId>wiremock-jre8</artifactId>