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/security/qa/service-account/src/javaRestTest/java/org/elasticsearch/xpack/security/authc/service/ServiceAccountIT.java
assertThat(apiKey.get("realm"), equalTo("_service_account"));
        assertThat(apiKey.get("invalidated"), is(invalidated));
    }
    private void assertServiceAccountRoleDescriptor(Response response, String serviceAccountPrincipal, String roleDescriptorString)
        throws IOException {
        final Map<String, Object> responseMap = responseAsMap(response);
        assertThat(
            responseMap,
            hasEntry(
                serviceAccountPrincipal,
                Map.of("role_descriptor", XContentHelper.convertToMap(new BytesArray(roleDescriptorString), false, XContentType.JSON).v2())
            )
        );
    }
    @SuppressWarnings("unchecked")
    private void assertNodesCredentials(Map<String, Object> responseMap) {
        final Map<String, Object> nodes = (Map<String, Object>) responseMap.get("nodes_credentials");
        assertThat(nodes, hasKey("_nodes"));
        final Map<String, Object> header = (Map<String, Object>) nodes.get("_nodes");
        assertThat(header.get("total"), equalTo(2));
        assertThat(header.get("successful"), equalTo(2));
        assertThat(header.get("failed"), equalTo(0));
        assertThat(header.get("failures"), nullValue());
        final Map<String, Object> fileTokens = (Map<String, Object>) nodes.get("file_tokens");
        assertThat(fileTokens, hasKey("token1"));
        final Map<String, Object> token1 = (Map<String, Object>) fileTokens.get("token1");
        assertThat((List<String>) token1.get("nodes"), equalTo(List.of("test-cluster-0", "test-cluster-1")));
    }
}