…#146)

* feat(images): implement authenticated gRPC redirection for Images API

- Intercept `execute_transforms`, `histogram`, and `composite` calls in the SDK.
- Redirect these calls to a custom, self-hosted gRPC service defined by `IMAGES_SERVICE_ENDPOINT`.
- Implement secure service-to-service authentication using OIDC tokens (via `google.auth` and `grpc.access_token_call_credentials`).
- Support native Google Cloud Storage (GCS) integration by passing raw `/gs/` paths to the external service, bypassing legacy BlobKey encoding.
- Implement fallback logic for legacy BlobKeys: automatically fetch blob content locally and send raw bytes to the external service when a direct reference cannot be resolved.
- Increase gRPC max receive message size to 32MB to support high-resolution image processing.
- Refactor internal logic to share authentication and channel creation code across all API methods.

* ci: Add fail-fast: false to matrix strategy

This will ensure that the workflow continues to run for all Python versions even if one of them fails.

* ci: Remove deprecated Python 3.7 and 3.8 from matrix

These runtimes are deprecated as per the support schedule:

https://docs.cloud.google.com/appengine/docs/standard/lifecycle/support-schedule#python

* Update image-server configuration environment variable names.

* fix parsing value of USE_CUSTOM_IMAGES_GRPC_SERVICE

Set version tag to v3.0.0b1 for beta release