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
static final String BASIC_AUTH_VALUE = basicAuthHeaderValue(
        "x_pack_rest_user",
        SecuritySettingsSourceField.TEST_PASSWORD_SECURE_STRING
    );
    @Override
    protected Settings restClientSettings() {
        return Settings.builder()
            .put(ThreadContext.PREFIX + ".Authorization", BASIC_AUTH_VALUE)
            .put(ThreadContext.PREFIX + ".X-elastic-product-origin", "fleet")
            .build();
    }
    public void testSearchWithoutIndexCreatedIsAllowed() throws Exception {
        Request request = new Request("GET", ".fleet-agents/_search");
        request.setJsonEntity("{ \"query\": { \"match_all\": {} } }");
        request.addParameter("ignore_unavailable", Boolean.TRUE.toString());
        assertEquals(200, client().performRequest(request).getStatusLine().getStatusCode());
    }
    public void testCreationOfFleetAgents() throws Exception {
        Request request = new Request("PUT", ".fleet-agents");
        Response response = client().performRequest(request);
        assertEquals(200, response.getStatusLine().getStatusCode());
        request = new Request("GET", ".fleet-agents/_mapping");
        response = client().performRequest(request);
        String responseBody = EntityUtils.toString(response.getEntity());
        assertThat(responseBody, containsString("access_api_key_id"));
        request = new Request("GET", ".fleet-agents-7/_mapping");
        response = client().performRequest(request);
        responseBody = EntityUtils.toString(response.getEntity());
        assertThat(responseBody, containsString("access_api_key_id"));