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
"Authorization",
                    basicAuthHeaderValue("elastic/fleet-server", new SecureString("x-pack-test-password".toCharArray()))
                )
        );
        final Response response = client().performRequest(request);
        assertOK(response);
        final Map<String, Object> responseMap = responseAsMap(response);
        assertThat(responseMap.get("username"), equalTo("elastic/fleet-server"));
        assertThat(responseMap.get("authentication_type"), equalTo("realm"));
        assertThat(responseMap.get("roles"), equalTo(List.of("superuser")));
        Map<?, ?> authRealm = (Map<?, ?>) responseMap.get("authentication_realm");
        assertThat(authRealm, hasEntry("type", "file"));
    }
    public void testCreateApiServiceAccountTokenAndAuthenticateWithIt() throws IOException {
        final Request createTokenRequest = new Request("POST", "_security/service/elastic/fleet-server/credential/token/api-token-1");
        final Response createTokenResponse = client().performRequest(createTokenRequest);
        assertOK(createTokenResponse);
        final Map<String, Object> createTokenResponseMap = responseAsMap(createTokenResponse);
        assertThat(createTokenResponseMap.get("created"), is(true));
        @SuppressWarnings("unchecked")
        final Map<String, String> tokenMap = (Map<String, String>) createTokenResponseMap.get("token");
        assertThat(tokenMap.get("name"), equalTo("api-token-1"));
        final Request request = new Request("GET", "_security/_authenticate");
        request.setOptions(RequestOptions.DEFAULT.toBuilder().addHeader("Authorization", "Bearer " + tokenMap.get("value")));
        final Response response = client().performRequest(request);
        assertOK(response);
        assertThat(
            responseAsMap(response),
            equalTo(
                XContentHelper.convertToMap(