Directpath support on non prod environments

## Issue Details

Directpath does not work unless a google universe domain is in the URL. However, there are cases where directpath can be used in non-prod environments.

https://github.com/googleapis/google-cloud-java/blob/main/sdk-platform-java/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java#L877.

Can we override [this check](https://github.com/googleapis/google-cloud-java/blob/main/sdk-platform-java/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java#L540) with a option or environment variable so DP is available in non-prod?

See https://b.corp.google.com/issues/479544840#comment6 for details

從 sdk-platform-java/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java 開始，檢查第 540 行和第 877 行附近與 DirectPath 相關的邏輯。判斷如何透過選項或環境變數覆寫 Google universe domain 檢查，然後驗證 DirectPath 在受支援的非生產環境中可用，同時不變更現有的生產行為。

由索引模型根據 Issue 內容生成。