Examples:

iex> body = %ApplePay.InitiateSessionBody{
...>   merchant_identifier: "merchant.test.payment.airatel.com",
...>   domain_name: "lpwpjtywvd.localtunnel.me",
...>   display_name: "ACME Shop"
...> }
iex> config = %ApplePay.Configuration{
...>   key: "priv/test/private.pem",
...>   cert: "priv/test/cert.pem",
...>   initiate_session_body: body
...> }
iex> url = "https://apple-pay-gateway-cert.apple.com/paymentservices/startSession" # Retrieved from Client
iex> ApplePay.initiate_session(config, url)
%{...}