Add TPC (Trusted Partner Cloud) guidance for Compute SSH

[VladaZakharova]

Document how to configure Compute Engine SSH in Trusted Partner Cloud environments.

This updates the Compute SSH connection and operator docs to clarify:

• which hook parameters to use for direct SSH in TPC
• which hook parameters to use for SSH over IAP in TPC
• that IAP tunneling requires the IAP-secured Tunnel User role
• that metadata-managed SSH keys should be used in the tested TPC environment instead of OS Login

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