Add configurable immediate server switching within a group

[simplecloud-vercel-github]

Problem

When switching servers within a server group, the next server does not start until the stopped server is fully offline. This delays the switch.

Expected behavior

Provide a configurable option that starts the target server immediately when the current server is stopped, without waiting for the current server to reach the fully offline state.

Acceptance criteria

• A server group can be configured to enable or disable immediate switching.
• When enabled, stopping one server and switching to another starts the target server immediately.
• When disabled, the existing behavior of waiting until the current server is fully offline is preserved.
• The configured behavior is applied consistently to server switches within the group.