static AccessTokenRequest ofClientCredentials(String clientId, String clientSecret) {
        return ofClientCredentials(ClientAuthentication.ofClientPassword(clientId, clientSecret));
    }
Creates a new {@link AccessTokenRequest} that is used to request an access token using the client ID
and client secret, as per
<a href="https://datatracker.ietf.org/doc/html/rfc6749#section-4.4">RFC 6749, Section 4.4</a>.
The specified client ID and client secret will be encoded and sent as the HTTP Basic authentication
header.
oauth2/src/main/java/com/linecorp/armeria/client/auth/oauth2/AccessTokenRequest.java