Add support for gRPC java

[shubhwip]

closes #243

Important Notes :

• Had to upgrade gradle from 4.9 to 5.6 to add gRPC dependencies which led to some changes in plugins declaration in gradle build file.
• Not sure how to keep these compiler flags with new gradle version as didn't find documentation for these flags anywhere

    it.options.compilerArgs += ["-XepAllDisabledChecksAsWarnings", "-XepDisableWarningsInGeneratedCode"]
    it.options.compilerArgs += ["-Xep:MutableMethodReturnType:OFF"]
    it.options.compilerArgs += ["-Xep:ReturnMissingNullable:OFF"]
    it.options.compilerArgs += ["-Xep:Var:OFF"]