Support Workload Identity Federation for GCS without requiring explicit credentials in cube/cubestore Docker image

**Describe the bug**
The `cube/cubestore` Docker image currently requires either `CUBESTORE_GCP_CREDENTIALS` or `CUBESTORE_GCP_KEY_FILE` to be set when using `CUBESTORE_GCS_BUCKET`. This makes it impossible to use [Workload Identity Federation](https://cloud.google.com/iam/docs/workload-identity-federation) (WIF) for authentication with GCS, which is the recommended approach in many Kubernetes-based and multi-cloud environments.

**To Reproduce**
Steps to reproduce the behavior:
1. Deploy `cube/cubestore` in a GCP-compliant environment using WIF (e.g., with a federated identity set up via GKE or another compliant setup).
2. Set only the `CUBESTORE_GCS_BUCKET` environment variable.
3. Start the container.
4. The application fails to authenticate with GCS due to missing `CUBESTORE_GCP_CREDENTIALS` or `CUBESTORE_GCP_KEY_FILE`.

**Expected behavior**
`cube/cubestore` should allow authentication to GCS using the default application credentials provided by the GCP environment (i.e., WIF or ADC), without requiring explicit key files or inline credentials, as long as the environment is correctly configured.

**Screenshots**
N/A

**Minimally reproducible Cube Schema**
_Not relevant for this issue, as it's Docker/environment related._

**Version:**
e.g. `cube/cubestore:v1.3`

**Additional context**
It would be great if the Docker image could support Workload Identity Federation by making `CUBESTORE_GCP_CREDENTIALS` and `CUBESTORE_GCP_KEY_FILE` optional when `CUBESTORE_GCS_BUCKET` is set. This would align with GCP best practices and improve security by avoiding service account key files.

A possible approach would be to rely on [Google's ADC (Application Default Credentials)](https://cloud.google.com/docs/authentication/application-default-credentials) chain, which is compatible with Workload Identity.