@SuppressWarnings("unchecked")
    public final T clientCredentials(
            Supplier<? extends Map.Entry<String, String>> clientCredentials, String authorizationType) {
        if (clientAuthorization != null) {
            throw new IllegalStateException("either client authorization or client credentials already set");
        }
        clientAuthorization = ClientAuthorization.ofCredentials(clientCredentials, authorizationType);
        return (T) this;
    }
Provides client authorization for the OAuth 2.0 requests based on client credentials and
authorization type,
as per <a href="https://datatracker.ietf.org/doc/html/rfc6749#section-2.3">[RFC6749], Section 2.3</a>.

@param clientCredentials A supplier of client credentials.
@param authorizationType One of the registered HTTP authentication schemes as per
                         <a href="https://www.iana.org/assignments/http-authschemes/http-authschemes.xhtml">
                         HTTP Authentication Scheme Registry</a>.
@throws IllegalStateException if clientCredentials already set
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/AbstractOAuth2AuthorizationGrantBuilder.java