code: public static void addGlobalMappingFilter(String filterSpec) {
          List<String> list = S.fastSplit(filterSpec, ",");
          for (String s : list) {
              if (S.blank(s)) {
                  continue;
              }
              addSingleGlobalMappingFilter(s.trim());
          }
      }
docs: Register a global mapping filter. Unlike normal mapping filter, global mapping filter spec
  
  * checks only immediate level field name or map key
  * it is always a black list filter
  * it supports different matching verb:
  - `contains:<payload>` - if test string contains `payload` then it shall be ignored
  - `starts:<payload>` - if test string starts with `payload` then it shall be ignored
  - `ends:<payload>` - if test string ends with `payload` then it shall be ignored
  - `reg:<payload>` - if test string matches reg exp pattern `payload` then it shall be ignored
  * if the filter spec dose not fall in the above category, then it is added into a set, and
  if any test string exactly matches any item in that set, it shall be ignored.
  
  @param filterSpec
  the filter spec. It can contains multiple filter specs separated by `,`