[bug] `pivot.ssh_exec` does not handle stderr

**Describe the bug**
pivot.ssh_exec does not handle stderr. Im pretty sure that russh exposes this in the "extended_data" on the channel

https://github.com/Eugeny/russh/blob/1d7e52fb710f5d5c505ed63733ecec4a2b96207c/russh/src/server/mod.rs#L416
