Allow for an alternate ssh command

[hmadison]

Description

For example, when using Teleport as an ssh proxy, it would be nice to be able to use their ssh command to access nodes on various trusted clusters.

[zenspider]

I'm not sure this request makes sense given that we don't call out to ssh and instead speak the ssh protocol directly. Assigning to @clintoncwolfe to weigh in on that and @kekaichinose to prioritize (assuming it is possible at all).