public OAuth2AuthorizationGrantBuilder accessTokenRequest(
            Supplier<AccessTokenRequest> accessTokenRequestSupplier) {
        requireNonNull(accessTokenRequestSupplier, "accessTokenRequestSupplier");
        requestSupplier = accessTokenRequestSupplier;
        return this;
    }
Sets the {@link Supplier} of {@link AccessTokenRequest} to be used when requesting an access token.
The supplier will be invoked every time when an access token is requested.

<p>Either an {@link AccessTokenRequest} or the {@link Supplier} of an {@link AccessTokenRequest}
must be set before building an {@link OAuth2AuthorizationGrant}.
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/OAuth2AuthorizationGrantBuilder.java