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
.privileges("view_index_metadata")
                            .allowRestrictedIndices(true)
                            .build() },
                    null,
                    null,
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA,
                    null
                )
            )
            .put(
                "enrich_user",
                new RoleDescriptor(
                    "enrich_user",
                    new String[] { "manage_enrich", "manage_ingest_pipelines", "monitor" },
                    new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder().indices(".enrich-*").privileges("manage", "read", "write").build() },
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA
                )
            )
            .put("viewer", buildViewerRoleDescriptor())
            .put("editor", buildEditorRoleDescriptor())
            .immutableMap();
    }
    private static RoleDescriptor buildViewerRoleDescriptor() {
        return new RoleDescriptor(
            "viewer",
            new String[] {},
            new RoleDescriptor.IndicesPrivileges[] {