Part of https://github.com/elastic/kibana/issues/154715

The `.fleet-secrets` system index should just have a value key. 

Kibana system should only have write and delete access NOT read access.

Fleet server will have read access only. 

Example document:

```
// example document from the .fleet-secrets index 
{
	_id: 'abcd1234'
	_source: {
	   value: "abcd12345678",
      }
}
```
 
x-pack/plugin/fleet/src/main/java/org/elasticsearch/xpack/fleet/Fleet.java
ScriptService scriptService,
        NamedXContentRegistry xContentRegistry,
        Environment environment,
        NodeEnvironment nodeEnvironment,
        NamedWriteableRegistry namedWriteableRegistry,
        IndexNameExpressionResolver expressionResolver,
        Supplier<RepositoriesService> repositoriesServiceSupplier,
        Tracer tracer,
        AllocationService allocationService
    ) {
        FleetTemplateRegistry registry = new FleetTemplateRegistry(
            environment.settings(),
            clusterService,
            threadPool,
            client,
            xContentRegistry
        );
        registry.initialize();
        return List.of();
    }
    @Override
    public Collection<SystemIndexDescriptor> getSystemIndexDescriptors(Settings settings) {
        return List.of(
            fleetActionsSystemIndexDescriptor(),
            fleetAgentsSystemIndexDescriptor(),
            fleetEnrollmentApiKeysSystemIndexDescriptor(),
            fleetPoliciesSystemIndexDescriptor(),
            fleetPoliciesLeaderSystemIndexDescriptor(),
            fleetServersSystemIndexDescriptors(),
            fleetArtifactsSystemIndexDescriptors()