Make guava an optional dependency of java-driver-guava-shaded

With CASSJAVA-52, the java-driver-guava-shaded is now in tree.

This appears to work great, but there is a slight issue with the
dependency tree that allows unshaded guava packages to be imported
within the project.

It looks like marking guava as an optional dependency in
java-driver-guava-shaded resolves this.

patch by Andy Tolbert; reviewed by Alexandre Dutra and Dmitry Konstantinov for CASSJAVA-76

Author: tolbertam

guava-shaded/pom.xml

@@ -46,6 +46,7 @@
           <artifactId>error_prone_annotations</artifactId>
         </exclusion>
       </exclusions>
+      <optional>true</optional>
     </dependency>
     <dependency>
       <groupId>org.graalvm.nativeimage</groupId>