[Java] feature request : Add --java-package-suffix option to flatc

[hhowe29]

I would like to bury generated files one level down to get them away from hand written source code files in the same package. So given an fbs file with this namespace declaration

// filename bar.fbs
namespace com.awesomecompany.foo;

I would like to run

flatc -o ./src/main/java/ --java  --java-package-suffix dto bar.fbs

and have the java files wind up in src/main/java/com.awesomecompany/foo/dto

I know I could just add dto to the namespace in the fbs file, but that would impact other languages in ways that I might not want.