docs: public final FeatureSource<?, ?> getFeatureSource(
              @Nonnull final MfClientHttpRequestFactory httpRequestFactory,
              @Nonnull final MapfishMapContext mapContext) {
          if (this.featureSource == null) {
              this.featureSource = this.featureSourceSupplier.load(httpRequestFactory, mapContext);
          }
          return this.featureSource;
      }