claims: add JSON serialization for interface arrays

[othman-essabir]

The current implementation lacks support for parsing and handling array structures in OIDC claims, which severely limits SSO integration with OIDC providers like Keycloak. Previously, array values in OIDC claims were raising "converting claim" errors. The parsing failed to properly map user roles from array-based claims.

Description

• Added type case for []interface{} that marshals arrays to JSON string format.
• Enables proper processing of multi-value OIDC claims such as roles.
• Maintains backward compatibility while extending support for array structures.

Testing & Reproduction Steps

Before fix

Trying to map the roles claim results in a 500 error:

http: request failed: method=POST path=/v1/acl/oidc/complete-auth \
error="error converting claim 'roles' to string from unknown type []interface {}" code=500

After fix

• The same configuration now properly processes array claims.
• Array values are serialized to JSON strings for consistent handling.
• User roles are correctly mapped from the OIDC provider.

Example of the processed claim:

internal_claims = {
  "roles": "[\"role1\",\"role2\",\"roleN\"]"
}

Changes to Security Controls

Yes, this PR includes changes to security controls:
Access Controls: Enables proper role mapping from OIDC providers, ensuring users receive correct authorization levels

[hashicorp-cla-app]

All committers have signed the CLA.