Related: #6724
Related: #7512

This PR sets the default SSH max timeout to a finite value. Without this setting, SSH connections can hang indefinitely, so in case of any failure, the connection is left as is and causes #7512.

Once we enable the timeout, coder agent will be able to report any troubles with broken agent connections:

2023-05-15 14:24:39.305 [debu]  ssh-server: copy output done  bytes=37025  error=EOF
2023-05-15 14:24:39.305 [warn]  ssh-server: ssh session failed ...
    error= copy error:
               github.com/coder/coder/agent/agentssh.(*Server).startPTYSession
                   /Users/mtojek/code/coder/agent/agentssh/agentssh.go:355
             - EOF

Otherwise, the agent assumes that the connection gets back to life eventually (hence no error trace in logs)