public OAuth2TokenDescriptorBuilder subject(String subject) {
        this.subject = requireNonNull(subject, "subject");
        return this;
    }
{@code sub} 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/OAuth2TokenDescriptorBuilder.java