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(e.getResponse().getStatusLine().getStatusCode(), equalTo(409));
        assertThat(e.getMessage(), containsString("document already exists"));
    }
    public void testGetServiceAccountCredentials() throws IOException {
        final Request getTokensRequest = new Request("GET", "_security/service/elastic/fleet-server/credential");
        final Response getTokensResponse1 = client().performRequest(getTokensRequest);
        assertOK(getTokensResponse1);
        final Map<String, Object> getTokensResponseMap1 = responseAsMap(getTokensResponse1);
        assertThat(getTokensResponseMap1.get("service_account"), equalTo("elastic/fleet-server"));
        assertThat(getTokensResponseMap1.get("count"), equalTo(1));
        assertThat(getTokensResponseMap1.get("tokens"), equalTo(Map.of()));
        assertNodesCredentials(getTokensResponseMap1);
        final Request createTokenRequest1 = new Request("POST", "_security/service/elastic/fleet-server/credential/token/api-token-1");
        final Response createTokenResponse1 = client().performRequest(createTokenRequest1);
        assertOK(createTokenResponse1);
        final Request createTokenRequest2 = new Request("POST", "_security/service/elastic/fleet-server/credential/token/api-token-2");
        final Response createTokenResponse2 = client().performRequest(createTokenRequest2);
        assertOK(createTokenResponse2);
        final Response getTokensResponse2 = client().performRequest(getTokensRequest);
        assertOK(getTokensResponse2);
        final Map<String, Object> getTokensResponseMap2 = responseAsMap(getTokensResponse2);
        assertThat(getTokensResponseMap2.get("service_account"), equalTo("elastic/fleet-server"));
        assertThat(getTokensResponseMap2.get("count"), equalTo(3));
        assertThat(getTokensResponseMap2.get("tokens"), equalTo(Map.of("api-token-1", Map.of(), "api-token-2", Map.of())));
        assertNodesCredentials(getTokensResponseMap2);
        final Request deleteTokenRequest1 = new Request("DELETE", "_security/service/elastic/fleet-server/credential/token/api-token-2");
        final Response deleteTokenResponse1 = client().performRequest(deleteTokenRequest1);
        assertOK(deleteTokenResponse1);
        assertThat(responseAsMap(deleteTokenResponse1).get("found"), is(true));
        final Response getTokensResponse3 = client().performRequest(getTokensRequest);