public OAuth2ResourceOwnerPasswordCredentialsGrantBuilder userCredentials(
            Supplier<? extends Entry<String, String>> userCredentials) {
        userCredentialsSupplier = requireNonNull(userCredentials, "userCredentials");
        return this;
    }
A supplier of user credentials: "username" and "password" used to grant the Access Token. REQUIRED.
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/OAuth2ResourceOwnerPasswordCredentialsGrantBuilder.java