code: public AsyncLookupInBuilder exists(String path, SubdocOptionsBuilder optionsBuilder) {
          if (path == null) {
              throw new IllegalArgumentException("Path is mandatory for subdoc exists");
          }
          if (optionsBuilder.createPath()) {
              throw new IllegalArgumentException("Options createPath are not supported for lookup");
          }
          this.specs.add(new LookupSpec(Lookup.EXIST, path, optionsBuilder));
          return this;
      }
docs: Check if a value exists inside the document (if it does not, attempting to get the
  {@link DocumentFragment#content(int)} will raise an error).
  This doesn't transmit the value on the wire if it exists, saving the corresponding byte overhead.
  
  @param path the path inside the document to check for existence.
  @param optionsBuilder {@link SubdocOptionsBuilder}
  @return this builder for chaining.