smart-release: support private registries?

[scullionw]

Summary 💡

unless i'm missing something, this is not supported yet (like cargo release --registry X)

Motivation 🔦

No response

[Byron]

It can theoretically support whatever cargo publish can support which is what will eventually be called.
If it's as easy as adding --registry X to the arguments of cargo publish, I think that could be quite a straightforward contribution.