improved SSH config with short-lived certificates

[profiprog]

This is suggestion to simplify client's SSH configuration for short-lived certificates.

I am aware that command cloudflared access ssh-config --hostname vm.example.com --short-lived-cert still generates old configuration - so I suggest to update also that if somebody known where.

• Old configuration is working only if client user is same as user of SSO identities.
• Old configuration does not works if passing remote command as argument to ssh command (for example ssh vm.example.com whoami)

This MR also includes the example of command to create new UNIX username as alias to existing one (which is useful if existing does not match user SSO identities).

[gitguardian[bot]]

⚠️ GitGuardian has uncovered 4 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	Secret	Commit	Filename
-	Generic High Entropy Secret	7d3d958ba7e1005d201a7e62289737fd5d716394	config.toml	View secret
-	Generic High Entropy Secret	58b6333afd7670da52bd237bec6b2c0cb708ae5e	products/ddos-protection/docs-config.js	View secret
-	Generic High Entropy Secret	f5ad72dcb5f9b6b4072be76c18ee663e52f30f72	products/ddos-protection/docs-config.js	View secret
3543783	Generic High Entropy Secret	51cbbada456648d933bd539e901cb20bb22d0adb	products/ruleset-engine/docs-config.js	View secret

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To avoid such incidents in the future consider

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[ranbel]

@jroyal Should we add this info into the docs?

[ranbel]

Closing due to inactivity and unresolved comments. We'll be glad to incorporate the suggested commands once they are verified to work and have more context added.