@SuppressWarnings("unchecked")
    public final T fallbackTokenProvider(
            Supplier<CompletableFuture<? extends GrantedOAuth2AccessToken>> fallbackTokenProvider) {
        delegate.fallbackTokenProvider(fallbackTokenProvider);
        return (T) this;
    }
An optional {@link Supplier} to acquire an access token before requesting it to the authorization server.
If the provided {@link GrantedOAuth2AccessToken} is valid, the client doesn't request a new token.

<p>This is supposed to be used with {@link #newTokenConsumer(Consumer)} and gets executed
in the following cases:
<ul>
    <li>Before the first attempt to acquire an access token.</li>
    <li>Before a subsequent attempt after token issue or refresh failure.</li>
</ul>
@see #newTokenConsumer(Consumer)
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/AbstractOAuth2AuthorizationGrantBuilder.java