VHDS Support

Mostly creating this issue to:

1. Get an understanding of any historical context. I see there is an open PR (https://github.com/envoyproxy/java-control-plane/pull/264) but it's stale.
2. Gauge if there is an interest in adding support to the java control plane

I am currently working on an envoy control plane on top of this repository and looking at what could be potentially improved, and this is at the top of the list.

首先檢視 issue 中引用的歷史背景和已過時的 PR #264，然後檢查 Java control plane 目前如何處理相關的 Envoy discovery services。該 issue 沒有定義具體的實作範圍或驗收標準，因此若要完成這項工作，首先需要就是否需要 VHDS 支援，以及它應包含哪些內容達成共識。

由索引模型根據 Issue 內容生成。