Support GCP IAM authentication for Redis

### Describe the problem

Allow the Weblate Helm chart to connect to Google Cloud Memorystore Redis using IAM authentication instead of static passwords. We could use a traditional username/password account which is not the best way to achieve that from google, we should use a service account for that (which has not password for this type of account)

### Solution brainstorm

Currently, the chart only supports standard Redis passwords. In GCP, Memorystore can now support IAM-based authentication for Redis, and the Python library [google-cloud-iam](https://pypi.org/project/google-cloud-iam/)
allows generating short-lived access tokens to connect. Here an [exemple](https://docs.cloud.google.com/memorystore/docs/cluster/client-library-connection#iam-auth-in-transit-encryption)

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_