chore: add mvn package to the makefile

Author: JenniferMah

Makefile

@@ -6,6 +6,9 @@ install:
 	mvn clean install -DskipTests=true -Dgpg.skip -Dmaven.javadoc.skip=true -B
 	cp target/sendgrid-java-$(VERSION)-shaded.jar sendgrid-java.jar
 
+package:
+	mvn package -DskipTests=true -Dgpg.skip -Dmaven.javadoc.skip=true -B
+
 test:
 	mvn test spotbugs:spotbugs checkstyle:check -Dcheckstyle.config.location=google_checks.xml