put back jetty-server as api dep

Author: amarziali

dd-java-agent/testing/build.gradle

@@ -53,7 +53,7 @@ dependencies {
   api deps.testLogging
   api deps.guava
 
-  implementation group: 'org.eclipse.jetty', name: 'jetty-server', version: '9.2.30.v20200428'
+  api group: 'org.eclipse.jetty', name: 'jetty-server', version: '9.2.30.v20200428'
   // ^ 9.2 last to support java 7
   api group: 'com.squareup.okhttp3', name: 'logging-interceptor', version: versions.okhttp_legacy