Add deferred badge loading for relation manager tabs

[buzkall]

Description

Extends the deferred tab badge loading introduced in #19336 to relation manager tabs.

Problem: RelationManager::getTabComponent() evaluates getBadge() synchronously, and the Tabs component built by HasRelationManagers has no key — both of which prevent deferBadge() from working.

Solution:

• Add protected static bool $deferBadge = false to RelationManager (overrideable via isBadgeDeferred() for dynamic, per-record control)
• Add fluent ->deferBadge() to RelationGroup
• Add ->key('relationManagerTabs') to the Tabs built in HasRelationManagers

Consumer usage

class TicketsRelationManager extends RelationManager
{
    protected static bool $deferBadge = true;

    public static function getBadge(Model $ownerRecord, string $pageClass): ?string
    {
        $count = $ownerRecord->tickets()->count();

        return $count > 0 ? (string) $count : null;
    }
}

No need to override getTabComponent() or the Edit page class.

Checklist

• Code style has been fixed by running the composer cs command.
• Changes have been tested — all 56 existing tests pass, 3 new tests added.
• Documentation is up-to-date.

[subIT1]

Wouldn't it make sence to add a defer option in addition to all badges. So TextColumns, NavigationBadge, TextEntry...