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/javaRestTest/java/org/elasticsearch/xpack/fleet/FleetSystemIndicesIT.java
assertEquals(200, response.getStatusLine().getStatusCode());
        request = new Request("GET", ".fleet-servers/_mapping");
        response = client().performRequest(request);
        String responseBody = EntityUtils.toString(response.getEntity());
        assertThat(responseBody, containsString("architecture"));
        request = new Request("GET", ".fleet-servers-7/_mapping");
        response = client().performRequest(request);
        responseBody = EntityUtils.toString(response.getEntity());
        assertThat(responseBody, containsString("architecture"));
    }
    public void testCreationOfFleetActionsResults() throws Exception {
        Request request = new Request("POST", "/.fleet-actions-results/_doc");
        request.setJsonEntity("{ \"@timestamp\": \"2099-03-08T11:06:07.000Z\", \"agent_id\": \"my-agent\" }");
        Response response = client().performRequest(request);
        assertEquals(201, response.getStatusLine().getStatusCode());
    }
    @SuppressWarnings("unchecked")
    public void verifyActionsILMPolicyExists() throws Exception {
        assertBusy(() -> {
            Request request = new Request("GET", "_ilm/policy/.fleet-actions-results-ilm-policy");
            Response response = client().performRequest(request);
            assertEquals(200, response.getStatusLine().getStatusCode());
            final String responseJson = EntityUtils.toString(response.getEntity());
            Map<String, Object> responseMap = XContentHelper.convertToMap(XContentType.JSON.xContent(), responseJson, false);
            assertNotNull(responseMap.get(".fleet-actions-results-ilm-policy"));
            Map<String, Object> policyMap = (Map<String, Object>) responseMap.get(".fleet-actions-results-ilm-policy");
            assertNotNull(policyMap);
            assertThat(policyMap.size(), equalTo(2));
        });
    }