[rust-launcher] Address TODO(#2479): configurable registry auth endpoint

[barakeinav1]

Context

Follow-up from PR #2326 (port node launcher to Rust). Tracked in #2598.

Problem

In crates/tee-launcher/src/main.rs (line 252), there is a TODO:

// TODO(#2479): if we use a different registry, we need a different auth-endpoint

The auth token URL is hardcoded to Docker Hub (auth.docker.io). The RegistryInfo trait exists but the auth endpoint is not configurable.

Action

If non-Docker-Hub registries are needed:

• Make the auth endpoint configurable via LauncherConfig
• Update the RegistryInfo trait implementation
• Add tests for alternative registries

Low priority unless private registries are planned.