@Nullable
    final ClientAuthentication buildClientAuthentication() {
        if (clientAuthorization == null) {
            return null;
        }
        return clientAuthorization.toClientAuthentication();
    }
Provides client authorization for the OAuth 2.0 requests based on client credentials and
{@code Basic} 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.
@throws IllegalStateException if clientCredentials already set
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/AbstractOAuth2AuthorizationGrantBuilder.java