Support non terminal environments with signer's sts providers

[puerco]

This pull request refactors and enhances the Carabiner authentication flow, with a focus on simplifying the login process and improving support for both CI and interactive environments. The main change is the introduction of a unified Login function that first attempts to use ambient credentials (like OIDC tokens from CI providers) and falls back to a browser-based login only when necessary. This makes the authentication process more robust, especially in automated environments, and reduces duplicated logic. The update also removes legacy GitHub Actions OIDC token retrieval code in favor of a more generic and extensible provider-based approach.

Key changes include:

Authentication Flow Refactor:

• Introduced a new Login function in pkg/client/credentials/login.go that:

  • Probes for ambient credentials using registered STS providers in a deterministic order and exchanges them for a Carabiner identity token if available.
  • Falls back to an interactive browser login only if no ambient credentials are found and the process is running in a terminal.
  • Provides clear error handling for non-interactive environments without ambient credentials.

• Removed the custom GitHub Actions OIDC token retriever (pkg/client/github/actions.go), consolidating ambient credential detection and retrieval under the new provider-based approach.

Command-Line Interface Updates:

• Updated the login command in pkg/cmd/login.go to use the new Login function, removing all inline logic for browser-based authentication and simplifying the command's flow. The help text and documentation were also updated to reflect the new behavior. [1] [2]

Testing Improvements:

• Added comprehensive unit tests for the new login logic in pkg/client/credentials/login_test.go, covering ambient credential detection, error handling, and fallback scenarios.

Dependency Updates:

• Updated dependencies in go.mod to support the new authentication providers and flows, including newer versions of carabiner-dev/signer, sigstore, and related libraries.