code: @Bean
      @ConditionalOnMissingBean(ChannelTokenSupplier.class)
      public ChannelTokenSupplier channelTokenSupplier() {
          final String channelToken = lineBotProperties.getChannelToken();
          return FixedChannelTokenSupplier.of(channelToken);
      }
docs: Expose {@link FixedChannelTokenSupplier} as {@link Bean}
  in case of no other definition for {@link ChannelTokenSupplier} type.