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/test/java/org/elasticsearch/xpack/fleet/FleetTests.java
public void testFleetIndexNames() {
        Fleet module = new Fleet();
        final Collection<SystemIndexDescriptor> fleetDescriptors = module.getSystemIndexDescriptors(Settings.EMPTY);
        assertThat(
            fleetDescriptors.stream().map(SystemIndexDescriptor::getIndexPattern).collect(Collectors.toList()),
            containsInAnyOrder(
                ".fleet-servers*",
                ".fleet-policies-[0-9]+*",
                ".fleet-agents*",
                ".fleet-actions~(-results*)",
                ".fleet-policies-leader*",
                ".fleet-enrollment-api-keys*",
                ".fleet-artifacts*"
            )
        );
        assertTrue(fleetDescriptors.stream().anyMatch(d -> d.matchesIndexPattern(".fleet-servers")));
        assertTrue(fleetDescriptors.stream().anyMatch(d -> d.matchesIndexPattern(".fleet-policies")));
        assertTrue(fleetDescriptors.stream().anyMatch(d -> d.matchesIndexPattern(".fleet-policies-leader")));
        assertTrue(fleetDescriptors.stream().anyMatch(d -> d.matchesIndexPattern(".fleet-agents")));
        assertTrue(fleetDescriptors.stream().anyMatch(d -> d.matchesIndexPattern(".fleet-actions")));
        assertFalse(fleetDescriptors.stream().anyMatch(d -> d.matchesIndexPattern(".fleet-actions-results")));
    }
    public void testFleetFeature() {
        Fleet module = new Fleet();
        Feature fleet = Feature.fromSystemIndexPlugin(module, Settings.EMPTY);
        SystemIndices systemIndices = new SystemIndices(List.of(fleet));
        assertNotNull(systemIndices);
    }
}