We'll want to access the context that was originally passed to us later
on in a future commit, so let's preserve it so we can extract it from
our API client.
In the future, we'll want to call into the SSH code from multiple
packages, so let's move it out of the lfshttp package into its own
package to avoid package import loops. While we're at it, rename the
function names to remove the "ssh" prefix, since it's implied by the
fact that they're in a package called "ssh".
Move the tests to their own package to prevent an import loop and expose
the private functions so we can test them there.
