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.