Issue #3554408: Include iss (issuer) claim in JWT tokens per RFC 8725

Mateu Aguiló Boschrequested to merge
issue/simple_oauth-3554408:3554408-include-iss-issuer into 6.0.x

Added default issuer claim to JWT tokens using the same approach as OpenID Connect module. The iss claim enhances token security as recommended by RFC 8725 Section 3.1. Custom iss claims from private_claims are still supported.

Added test coverage to verify the iss claim is properly included in tokens.

Closes #3554408

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