feat(auth): deprecate unsafe credentials JSON loading options

[quartzmo]

• Add deprecation comments to credentials.DetectOptions.CredentialsFile
  and credentials.DetectOptions.CredentialsJSON.
• Update documentation to recommend new safe load methods.
• Add credentials.CredentialsType.
• Add credentials.NewCredentialsFromJSON and credentials.NewCredentialsFromFile.
• Add deprecation comments to idtoken.Options.CredentialsFile
  and idtoken.Options.CredentialsJSON.
• Add idtoken.NewCredentialsFromJSON and idtoken.NewCredentialsFromFile.
• Replace internal integer-based CredentialsType (iota) with
  public string-based constants for specifying credential file
  types. This change:
  • Eliminates the zero-value problem
  • Improves readability and debugging
  • Ensures consistency with JSON data

[gemini-code-assist]

Summary of Changes

Hello @quartzmo, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly refactors how credential configurations are handled, primarily focusing on improving security and clarity. It introduces new, type-specific functions for loading credentials from files or JSON, which enforce type validation and provide explicit security warnings for certain credential types. Concurrently, older, less secure methods for loading credentials directly via DetectOptions and idtoken.Options fields have been deprecated, guiding users towards the new, safer alternatives. These changes aim to prevent accidental loading of malicious or unexpected credential types and promote best practices for handling sensitive authentication data.

Highlights

• Credential Type Standardization: Introduced a new public CredentialsType string enum to replace an internal integer-based CredentialType, improving readability, debugging, and consistency across credential handling.
• New Credential Loading Functions: Added credentials.NewCredentialsFromJSON and credentials.NewCredentialsFromFile for safer, type-specific loading of credentials, which includes validation against the expected credential type.
• Deprecation of Direct Credential Fields: Deprecated credentials.DetectOptions.CredentialsFile and CredentialsJSON, as well as idtoken.Options.CredentialsFile and CredentialsJSON, recommending the new type-specific loading functions due to potential security risks with untrusted sources.
• ID Token Credential Loading: Introduced idtoken.NewCredentialsFromJSON and idtoken.NewCredentialsFromFile to provide type-validated loading for ID token credentials, enhancing security for this specific credential type.
• Enhanced Security Guidance: Updated documentation and deprecation messages to strongly advise validating credential configurations from external sources to mitigate security risks associated with unvalidated inputs.

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[gemini-code-assist]

Code Review

This pull request introduces significant security improvements by deprecating unsafe credential loading methods and adding new, safer alternatives that require explicit credential type specification. The move from integer-based iota to string constants for credential types is a great change for readability and robustness. The new functions are well-documented with clear security warnings, and the deprecation messages for the old fields are very informative. The changes are supported by a good set of new tests. I have identified a couple of important issues regarding context.Context propagation in the new functions that should be addressed before merging.

[hongalex]

approved with comment nit