ssh - Default user known hosts file

[petersutter]

What this PR does / why we need it:

This PR sets default paths for known_hosts files for SSH connections to bastions and shoot nodes when none are provided, reducing host key conflicts from IP reuse and keeping the user's default SSH known_hosts file (~/.ssh/known_hosts) clean.

• --bastion-user-known-hosts-file default: <temp_dir>/garden/cache/<bastion_uid>/.ssh/known_hosts
• --node-user-known-hosts-file default: <garden_home_dir>/cache/<shoot_uid>/.ssh/known_hosts

Release note:

`ssh`: Default paths for `known_hosts` files are set for bastions and shoot nodes. Bastion keys are stored in temporary directories, while shoot node keys persist in the garden home directory.

The session directory has been moved to a `sessions` subfolder, changing from `<temp_dir>/garden/<session_id>` to `<temp_dir>/garden/sessions/<session_id>`. The current session will be migrated on the next run of a `gardenctl` command.

[grolu]

/lgtm

[holgerkoser]

/lgtm