@Nullable
    public String subject() {
        return subject;
    }
{@value OAuth2Constants#SUBJECT} Token Introspection Response field,
OPTIONAL. Subject of the token. Usually a machine-readable
identifier of the resource owner who authorized this token. As defined at
<a href="https://datatracker.ietf.org/doc/rfc7519/">[RFC7519]</a>.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/OAuth2TokenDescriptor.java