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.getMessage(), containsString("no such index [.security]"));
        }
    }
    public void testAuthenticateShouldWorkWithOAuthBearerToken() throws IOException {
        final Request oauthTokenRequest = new Request("POST", "_security/oauth2/token");
        oauthTokenRequest.setJsonEntity("{\"grant_type\":\"password\",\"username\":\"test_admin\",\"password\":\"x-pack-test-password\"}");
        final Response oauthTokenResponse = adminClient().performRequest(oauthTokenRequest);
        assertOK(oauthTokenResponse);
        final Map<String, Object> oauthTokenResponseMap = responseAsMap(oauthTokenResponse);
        final String accessToken = (String) oauthTokenResponseMap.get("access_token");
        final Request request = new Request("GET", "_security/_authenticate");
        request.setOptions(RequestOptions.DEFAULT.toBuilder().addHeader("Authorization", "Bearer " + accessToken));
        final Response response = adminClient().performRequest(request);
        assertOK(response);
        final Map<String, Object> responseMap = responseAsMap(response);
        assertThat(responseMap.get("username"), equalTo("test_admin"));
        assertThat(responseMap.get("authentication_type"), equalTo("token"));
        final String refreshToken = (String) oauthTokenResponseMap.get("refresh_token");
        final Request refreshTokenRequest = new Request("POST", "_security/oauth2/token");
        refreshTokenRequest.setJsonEntity(Strings.format("""
            {"grant_type":"refresh_token","refresh_token":"%s"}
            """, refreshToken));
        final Response refreshTokenResponse = adminClient().performRequest(refreshTokenRequest);
        assertOK(refreshTokenResponse);
    }
    public void testAuthenticateShouldDifferentiateBetweenNormalUserAndServiceAccount() throws IOException {
        final Request request = new Request("GET", "_security/_authenticate");
        request.setOptions(
            RequestOptions.DEFAULT.toBuilder()
                .addHeader(