Feature request: Allow remote hostname token for User option

[Kenneth Perronne]

Is there a specific reason the %h token isn't available to the User option?
I'm trying to configure my local ssh_config to connect to remote systems
through a CyberArk PSM proxy and it functions by taking several connection
parameters for the downstream destination as username elements. One of
those needed elements is the remote host, which is available on %h, but
that can't be provided to User.

CLI connection example: ssh vaultUser at targetUser#domain at targetmachine
@PSMProxy

Desired config example:
----------
Host secure-*
  User myuser at remoteuser#domain@%h
  HostName PSMProxy
---------

This would allow any connections to secure-servername to work without
needing static entries per-host. If there are any other options for how to
implement this I'd be very open to feedback.

Thank you,
Kenneth