private String composeAuthorizationString() {
        final String clientAuthorization;
        if (authorizationSupplier != null) {
            clientAuthorization = authorizationSupplier.get();
        } else if (credentialsSupplier != null) {
            final Map.Entry<String, String> clientCredentials = credentialsSupplier.get();
            clientAuthorization = encodeClientCredentials(clientCredentials.getKey(),
                                                          clientCredentials.getValue());
        } else {
            // we should not get here
            throw new NullPointerException("authorizationSupplier && credentialsSupplier");
        }
        return clientAuthorization;
    }
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 credentialsSupplier A supplier of client credentials.

@deprecated Use {@link ClientAuthentication#ofClientPassword(String, String)} instead.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/ClientAuthorization.java