Support long lived sessions that can survive a disconnect

[SteveL-MSFT]

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Summary of the new feature / enhancement

On Linux, you can use something like tmux to keep a long lived session, but the equivalent doesn't exist on Windows.

Proposed technical implementation details (optional)

I think how this could work as a new exe for a sshd subsystem that can connect to a registered PowerShell session (either create a new one or connect to existing one). The PowerShell process would have to be decoupled from the sshd created job object and we would probably also need configuration for life of the session so it's not kept up perpetually.