OIDC audience validation skipped when --authn-oidc-audience is unset

Summary

OpenFGA is an authorization/permission engine built for developers. Prior to 1.18.0, OpenFGA's OIDC authenticator skipped JWT audience validation when authn.method was set to oidc, authn.oidc.issuer was configured, and authn.oidc.audience was not set, allowing a token minted for an unrelated service by the same identity provider to authenticate to OpenFGA. This issue is fixed in 1.18.0. Prior to 1.18.0, when OpenFGA is configured to use OIDC authentication (authn.method=oidc, authn.oidc.issuer set) but authn.oidc.audience is left unset, the JWT audience claim on incoming bearer tokens is not validated. As a result, a validly-signed OIDC access token issued by the same identity provider for a completely different, unrelated application can be accepted by OpenFGA as a valid credential, allowing an attacker holding such a token to authenticate to OpenFGA and perform unauthorized authorization queries or writes. An attacker who can obtain such a token, for example one issued to a lower-privileged or unrelated application at the same IdP, could use it to authenticate to OpenFGA and perform authorization queries or writes they should not be permitted to make. This flaw only affects deployments that explicitly enable OpenFGA's OIDC authentication method and leave --authn-oidc-audience unset.