Feature: Implement Issuer Validation for OpenID Connect Discovery document

This PR proposes to enhance the security of angular-auth-oidc-client by implementing a validation check for the issuer field in the OpenID Connect discovery document.

Problem:

During a recent internal review, we identified a scenario where a misconfigured reverse proxy could allow a cache poisoning attack. This attack could lead to a modified .well-known/openid-configuration payload being served from a malicious domain (e.g., evil.xyz.com), even when the configured authority is a legitimate Identity Provider (e.g., idp.corporate.com).

Currently, angular-auth-oidc-client accepts this malicious metadata without validating that the issuer field within the discovery document matches the expected authority.

Proposed Solution:

According to the OpenID Connect Discovery specification (Section 4.3):

"The issuer value returned MUST be identical to the Issuer URL that was used as the prefix to /.well-known/openid-configuration to retrieve the configuration information.”

This PR aims to implement this crucial validation. By verifying that the issuer value returned in the discovery document precisely matches the authority configured in the client, we can provide a significant defense-in-depth layer against cache poisoning and similar attacks.

Benefits:

• Enhanced Security: Prevents the library from accepting forged or malicious OpenID Connect configurations.
• Specification Compliance: Ensures the library adheres strictly to the OpenID Connect Discovery specification.
• Defense-in-Depth: Adds an important security control, even if upstream infrastructure issues are present.

Workaround :

Using the authWellknownEndpoints configuration option to explicitly pin the well-known endpoint URLs. However, we believe this check would benefit all users of the library as a standard security measure.