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
final Request getUserRequest = new Request("GET", "_security/user");
        getUserRequest.setOptions(getUserRequest.getOptions().toBuilder().addHeader("Authorization", "ApiKey " + base64ApiKeyKeyValue));
        final ResponseException e = expectThrows(ResponseException.class, () -> client().performRequest(getUserRequest));
        assertThat(e.getResponse().getStatusLine().getStatusCode(), equalTo(403));
        assertThat(e.getMessage(), containsString("is unauthorized for API key"));
        final Request invalidateApiKeysRequest = new Request("DELETE", "_security/api_key");
        invalidateApiKeysRequest.setJsonEntity(Strings.format("""
            {"ids":["%s"],"owner":true}""", apiKeyId1));
        invalidateApiKeysRequest.setOptions(requestOptions);
        final Response invalidateApiKeysResponse = client().performRequest(invalidateApiKeysRequest);
        assertOK(invalidateApiKeysResponse);
        final Map<String, Object> invalidateApiKeysResponseMap = responseAsMap(invalidateApiKeysResponse);
        assertThat(invalidateApiKeysResponseMap.get("invalidated_api_keys"), equalTo(List.of(apiKeyId1)));
        assertApiKeys(apiKeyId1, "key-1", true, requestOptions);
    }
    private void assertApiKeys(String apiKeyId, String name, boolean invalidated, RequestOptions.Builder requestOptions)
        throws IOException {
        final Request getApiKeysRequest = new Request("GET", "_security/api_key?owner=true");
        getApiKeysRequest.setOptions(requestOptions);
        final Response getApiKeysResponse = client().performRequest(getApiKeysRequest);
        assertOK(getApiKeysResponse);
        final Map<String, Object> getApiKeysResponseMap = responseAsMap(getApiKeysResponse);
        @SuppressWarnings("unchecked")
        final List<Map<String, Object>> apiKeys = (List<Map<String, Object>>) getApiKeysResponseMap.get("api_keys");
        assertThat(apiKeys.size(), equalTo(1));
        final Map<String, Object> apiKey = apiKeys.get(0);
        assertThat(apiKey.get("id"), equalTo(apiKeyId));
        assertThat(apiKey.get("name"), equalTo(name));
        assertThat(apiKey.get("username"), equalTo("elastic/fleet-server"));