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/core/src/main/java/org/elasticsearch/xpack/core/security/authz/store/ReservedRolesStore.java
new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices(Watch.INDEX, TriggeredWatchStoreField.INDEX_NAME, HistoryStoreField.INDEX_PREFIX + "*")
                            .privileges("read")
                            .allowRestrictedIndices(true)
                            .build() },
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA
                )
            )
            .put(
                "watcher_user",
                new RoleDescriptor(
                    "watcher_user",
                    new String[] { "monitor_watcher" },
                    new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices(Watch.INDEX)
                            .privileges("read")
                            .allowRestrictedIndices(true)
                            .build(),
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices(HistoryStoreField.INDEX_PREFIX + "*")
                            .privileges("read")
                            .build() },
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA
                )
            )
            .put(