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

Start in sdk-platform-java/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java, reviewing the DirectPath-related logic around lines 540 and 877. Determine how an option or environment variable could override the Google universe domain check, then verify that DirectPath is available for supported non-production environments without changing the existing production behavior.

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