VertexFeatureStore Support Missing in backend_config for Vertex AI Feature Store

**Title:** Regression: VertexFeatureStore Support Missing in backend_config for Vertex AI Feature Store

**Description:**
We encountered an issue while integrating `backend_config` for Vertex AI Feature Store. In a previous version, it supported `vectordb`, which included support for `VertexFeatureStore`. However, in the current version, `backend_config` no longer includes support for `VertexFeatureStore`.

This regression is affecting our workflow, as we relied on `VertexFeatureStore` support through `vectordb`. Could you confirm if this is an intended change or an oversight? If intended, is there an alternative way to configure `VertexFeatureStore` with `backend_config`?

**Expected Behavior:**
- `backend_config` should support `VertexFeatureStore`, as it did in the previous version.

**Current Behavior:**
- `VertexFeatureStore` is not supported in `backend_config` in the latest version.

**Steps to Reproduce:**
1. Attempt to configure `backend_config` with `VertexFeatureStore`.
2. Observe that it is no longer supported.

**Additional Context:**
Any additional details that may help diagnose the issue.

**Request:**
Please provide guidance on whether `VertexFeatureStore` support will be reintroduced or suggest an alternative approach.

まず、VertexFeatureStore を使用した backend_config の設定を再現し、vectordb を介してこれをサポートしていた以前のバージョンと動作を比較します。backend_config のエントリポイントを追跡し、VertexFeatureStore のサポートを復元できるか、または文書化された代替設定が必要かを判断します。サポートされる動作または代替手段が確認された時点で、issue は完了です。

索引モデルが issue の本文から書いたものです。