loadCatchpointFileHeaderFromRelay opens a streaming HTTP connection to the given relay for the given round, then scans the (possibly gzip) tar stream until it finds `content.json`, decodes the ledger.CatchpointFileHeader, and immediately closes the network connection (so we don't download the entire
(relay string, netName string, round int)
source not stored for this graph (policy: none)
no test coverage detected