public OAuth2TokenIntrospectionAuthorizerBuilder cacheSpec(String cacheSpec) {
        this.cacheSpec = CaffeineSpec.parse(cacheSpec); // parse right away
        return this;
    }
Provides caching facility for OAuth 2.0 {@link OAuth2TokenDescriptor} in order to avoid continuous Token
Introspection as per <a href="https://datatracker.ietf.org/doc/html/rfc7662#section-2.2">[RFC7662], Section 2.2</a>.
Sets the {@linkplain CaffeineSpec Caffeine specification string} of the cache that stores the tokens.
If not set, {@value DEFAULT_CACHE_SPEC} is used by default.
oauth2/src/main/java/com/linecorp/armeria/server/auth/oauth2/OAuth2TokenIntrospectionAuthorizerBuilder.java