Add support for `rustCustomDerive` and `rustCustomUse` metadata#31

AdamLeyshon wants to merge 2 commits intojsontypedef:masterjsontypedef/json-typedef-codegen:masterfrom

AdamLeyshon:feature/add_custom_use_and_deriveAdamLeyshon/json-typedef-codegen:feature/add_custom_use_and_deriveCopy head branch name to clipboard