change shell script to executable on Linux and Mac

[stevehu]

In the generated projects, we have some shell scripts that are supposed to be executables on Linux or Mac. For example, build.sh and gradlew etc. We need to add a method in the codegen to update the mode of the file after generation. Otherwise, we have to ask the user to run chmod +x gradlew in the generated project before calling the shell script.

[stevehu]

Along with the executable fix for mvnw and gradlew, I have moved the maven and gradle wrapper to separate folders in the resources and created two default transfer methods in the generator.