buildscript: iterate example folder and build

[YifeiZhuang]

cc. @temawi

[YifeiZhuang]

It seems revealing failure on example-servlet that was not covered by the current build.

+ pushd examples/example-servlet
+ ../gradlew build -PtargetArch=x86_64 -Pcheckstyle.ignoreFailures=false -PfailOnWarnings=true -PerrorProne=true -PskipAndroid=true -Dorg.gradle.parallel=true
/grpc-java
/grpc-java/examples/example-servlet /grpc-java

FAILURE: Build failed with an exception.

* Where:
Build file '/grpc-java/examples/example-servlet/build.gradle' line: 26

* What went wrong:
A problem occurred evaluating root project 'example-servlet'.
> Could not find method providedImplementation() for arguments [javax.servlet:javax.servlet-api:4.0.1, org.apache.tomcat:annotations-api:6.0.53] on object of type org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.