docs: public SignerConfig getSignerConfig(String serviceName, String regionName) {
          if (serviceName == null)
              throw new IllegalArgumentException();
          SignerConfig signerConfig = null;
          if (regionName != null) {
              // Service+Region signer config has the highest precedence
              String key = serviceName + SERVICE_REGION_DELIMITOR + regionName;
              signerConfig = serviceRegionSigners.get(key);
              if (signerConfig != null) {
                  return signerConfig;
              }
              // Region signer config has the 2nd highest precedence
              signerConfig = regionSigners.get(regionName);
              if (signerConfig != null) {
                  return signerConfig;
              }
          }
          // Service signer config has the 3rd highest precedence
          signerConfig = serviceSigners.get(serviceName);
          // Fall back to the default
          return signerConfig == null ? defaultSignerConfig : signerConfig;
      }