You can point it at any hub

Two new settings, both with working defaults, so there's nothing to configure: flWebsubEnabled is on, and urlWebsubHub is Andrew's hub at https://rpc.rsscloud.io/websub. Put a different address in config.json and both the announcement and the ping follow it. Set flWebsubEnabled to false and neither happens. One note for operators: the header rides on feeds the server itself serves, so if yours publishes to S3 instead, there's nothing to announce.