Make error_prone_annotations dependency optional

Annotations with retention policy are ignored if they cannot be loaded
at runtime. Make the dependency optional to not enforce the presence of
error_prone_annotations in a java runtime even if those annotations are
not used at all.

core/pom.xml

@@ -29,6 +29,7 @@
     <dependency>
       <groupId>com.google.errorprone</groupId>
       <artifactId>error_prone_annotations</artifactId>
+      <optional>true</optional>
     </dependency>
     <!--
      | ASM is embedded by default by the JarJar build profile