created DrugCategoryDTO and DrugCategoryDTOValidator

Signed-off-by: juliusmercons <julius.masanting@orangeandbronze.com>
modified DrugCategoryService to accept and return DTOs instead of Domains

Signed-off-by: juliusmercons <julius.masanting@orangeandbronze.com>
added messages_en.properties

Signed-off-by: juliusmercons <julius.masanting@orangeandbronze.com>