auth: Allow overriding the universeDomain using GOOGLE_CLOUD_UNIVERSE_DOMAIN

## Is your feature request related to a problem? Please describe.

Allow overriding the universe domain, similar to how the REST client allows overriding it using the environment variable `GOOGLE_CLOUD_UNIVERSE_DOMAIN`

The [getUniverseDomain()](https://github.com/googleapis/google-cloud-go/blob/c8ae2e2a0a89e378892414af4db41ccd9b40bc9e/auth/internal/transport/cba.go#L81-L86) call for the gRPC transport does not take into account the override environment variable.

https://github.com/googleapis/google-cloud-go/blob/c8ae2e2a0a89e378892414af4db41ccd9b40bc9e/auth/internal/transport/cba.go#L81-L86

## Describe the solution you'd like

Similar to the [REST implementation](https://github.com/googleapis/google-api-go-client/blob/62e5a8eeb37dc6d8b31b104bdb6f100369d69ff2/internal/settings.go#L251-L259) that respects the `GOOGLE_CLOUD_UNIVERSE_DOMAIN` environment variable to allow overriding the universeDomain.

## Describe alternatives you've considered
None at the moment, we are using a GCP whitelabel cloud provider which requires the universe domain to be overridden. At the moment, the only solution is to switch from gRPC to REST. However, many open source tools only implement the gRPC client and there is no solution at the moment.