Set seata core as optional dependency (#188)

diff --git a/dubbo-filter-extensions/dubbo-filter-seata/pom.xml b/dubbo-filter-extensions/dubbo-filter-seata/pom.xml
index 9f91200..71380f1 100644
--- a/dubbo-filter-extensions/dubbo-filter-seata/pom.xml
+++ b/dubbo-filter-extensions/dubbo-filter-seata/pom.xml
@@ -34,6 +34,7 @@
         <dependency>
             <groupId>io.seata</groupId>
             <artifactId>seata-core</artifactId>
+            <optional>true</optional>
         </dependency>
         <dependency>
             <groupId>org.apache.dubbo</groupId>