add OS TOPIC to set topic without spoofing setter

[jesopo]

like CS TOPIC but doesn't spoof the setter to the person that triggered ChanServ to set topic. a bit niche

[alyx]

Can you clarify the expected output in this case?

e.g. is the topic set by OperServ? ChanServ? other?

[jesopo]

not sure, though I think OperServ would make most sense