Add proxy support in sdk

[goldnorb]

Request: For our application, we need to call external applications like Voucherify via proxy.

Description: VoucherifyClient might be extended to optionally support a proxy server in OkHttpClient.

Example Code for OkHttpClient and Proxy: OkHttpClient client = new OkHttpClient.Builder() .proxy(new Proxy(Proxy.Type.HTTP, new InetSocketAddress(proxyHost, proxyPort))) .proxyAuthenticator(proxyAuthenticator) .build();

Please inform me, if this would be possible and approximately how long it will take. Thank you very much in advance.