CurrentUserMode with session, load known attributes within ActiveSession

What does this MR do and why?

The original MR has been reverted due to failures on staging-canary. See discussion !142158 (comment 1748221369) within !142158 (merged)

Note: As discussed this MR will be split into two pieces

1. This MR: Extend lib/gitlab/auth/current_user_mode.rb and add safe guard to only load known attributes within ActiveSession (admin_mode attribute)
2. Second MR: add admin_mode attribute, add view and usage of admin_mode attribute, revert of !143379 (46a8cec1)

initial description (before split into two)

Show admin mode within active sessions view

It is difficult to see if a session is within admin mode.

This MR adds the admin_mode to the active_user_session and extends the active sessions view to display which sessions are in admin mode.

Closes #438674 (closed)

Changelog: added

🛠 with ❤ at Siemens

NOTE

The original MR has been reverted due to failures on staging-canary. See discussion !142158 (comment 1748221369) within !142158 (merged)

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