bench,tv: factor out BroadcastConfig into a package so both oceantv and oceanbench can use it

This should be done so we don't have to maintain two versions of the struct. Perhaps we should create a config.go in cmd/oceantv/broadcast package.

Start by locating the two BroadcastConfig struct definitions used by oceantv and oceanbench. Review cmd/oceantv/broadcast/config.go as the proposed shared location, then update both consumers to use one definition. Done means the duplicate struct is removed and both commands build successfully.

Written by the indexing model from the issue text.