Move admin manager to shared

[ElectroJr]

About the PR

This PR moves various parts of ClientAdminManager and AdminManager into a new SharedAdminManager.

Why / Balance

Reduces code duplication and required for space-wizards/RobustToolbox#5862
This PR was spun off from #36717, removing any parts that relied on engine changes to try decouple the engine PR from content.

Technical details

Part of this PR changes IoC initialization logic a bit. In particular, the shared entity point used to inject dependencies in PreInit(). However, it now depends ISharedAdminManager which is registered by the server/client. So not it instead injects dependencies in Init(), while the client & server will register types in PreInit().

I think these changes aren't strictly required, and I could just not use a [Dependency] field, but I think these changes make sense and clear up initialization spaghetti a bit. But maybe this behavior is required for some reason?

Requirements

• I have read and am following the Pull Request and Changelog Guidelines.
• I have added media to this PR or it does not require an ingame showcase.

Breaking changes

• AdminCommandAttribute has been moved to shared (Content.Shared.Administration)
• Several methods in ClientAdminManager and AdminManager have modified or combined and moved to SharedAdminManager.
• Entrypoint initialization has changed slightly. In particular, the client & server will now perform IoC registrations (i.e., ServerContentIoC.Register()) in PreInit() instead of Init()