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
@Override
    protected String getProtocol() {
        
        return "https";
    }
    @Override
    protected Settings restAdminSettings() {
        final String token = basicAuthHeaderValue("test_admin", new SecureString("x-pack-test-password".toCharArray()));
        return Settings.builder().put(ThreadContext.PREFIX + ".Authorization", token).put(CERTIFICATE_AUTHORITIES, caPath).build();
    }
    @Override
    protected Settings restClientSettings() {
        final String token = basicAuthHeaderValue("service_account_manager", new SecureString("x-pack-test-password".toCharArray()));
        return Settings.builder().put(ThreadContext.PREFIX + ".Authorization", token).put(CERTIFICATE_AUTHORITIES, caPath).build();
    }
    public void testGetServiceAccount() throws IOException {
        final Request getServiceAccountRequest1 = new Request("GET", "_security/service");
        final Response getServiceAccountResponse1 = client().performRequest(getServiceAccountRequest1);
        assertOK(getServiceAccountResponse1);
        assertServiceAccountRoleDescriptor(getServiceAccountResponse1, "elastic/fleet-server", ELASTIC_FLEET_SERVER_ROLE_DESCRIPTOR);
        final Request getServiceAccountRequest2 = new Request("GET", "_security/service/elastic");
        final Response getServiceAccountResponse2 = client().performRequest(getServiceAccountRequest2);
        assertOK(getServiceAccountResponse2);
        assertServiceAccountRoleDescriptor(getServiceAccountResponse2, "elastic/fleet-server", ELASTIC_FLEET_SERVER_ROLE_DESCRIPTOR);
        final Request getServiceAccountRequest3 = new Request("GET", "_security/service/elastic/fleet-server");
        final Response getServiceAccountResponse3 = client().performRequest(getServiceAccountRequest3);
        assertOK(getServiceAccountResponse3);
        assertServiceAccountRoleDescriptor(getServiceAccountResponse3, "elastic/fleet-server", ELASTIC_FLEET_SERVER_ROLE_DESCRIPTOR);
        final Request getServiceAccountRequestKibana = new Request("GET", "_security/service/elastic/kibana");
        final Response getServiceAccountResponseKibana = client().performRequest(getServiceAccountRequestKibana);