build: explicitly map shade.skip in shaded module configurations

Maven Shade Plugin's skip parameter does not declare a built-in property expression in its Mojo descriptor, so explicit mapping via <skip>${shade.skip}</skip> is required for the profile property to take effect.

Author: lqiu96

java-spanner-jdbc/pom.xml

@@ -275,6 +275,7 @@
               <goal>shade</goal>
             </goals>
             <configuration>
+              <skip>${shade.skip}</skip>
               <createSourcesJar>true</createSourcesJar>
               <shadeSourcesContent>true</shadeSourcesContent>
               <shadedArtifactAttached>true</shadedArtifactAttached>

sdk-platform-java/gapic-generator-java/pom.xml

@@ -255,6 +255,7 @@
               <goal>shade</goal>
             </goals>
             <configuration>
+              <skip>${shade.skip}</skip>
               <createDependencyReducedPom>false</createDependencyReducedPom>
               <filters>
                 <filter>