Pro backend request/response glue code

[Doy-lee]

This PR goes ontop of #63 only need to review the delta between the two.

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The commits here are a polish pass over the draft code that was dumped into src/pro.cpp. That file has been renamed to src/pro_backend.cpp. The role of this file now is to interop with the pro backend and adds a bunch of functions to construct the necessary HTTP POST payloads to send off to a Session Pro Backend to do one of the following operations:

1. Redeem a payment the client has purchased for entitlement to Session Pro. This gets the Session Pro Backend after verifying it successfully, to generate a proof that the client can attach to their messages to enable Pro features.
2. Authorise a new Ed25519 key pair to be the signatory of their Session Pro Proof that can be used on the protocol
3. Retrieve the historical list of payments associated with a Session account
4. Get the current list of global proofs which is maintained by the Session Pro backend that clients will reject proofs from until the designated expiry time.

Additionally for each payment provider, libsession exposes the various platform specific URLs to centralise those strings into the same repository, as per:

const session_pro_payment_provider_metadata SESSION_PRO_PAYMENT_PROVIDER_METADATA[SESSION_PRO_PAYMENT_PROVIDER_COUNT] = {
    /*SESSION_PRO_PAYMENT_PROVIDER_NIL*/ {
        .request_refund_support_url = string8_literal(""),
        .subscription_page_url      = string8_literal(""),
    },
    /*SESSION_PRO_PAYMENT_PROVIDER_GOOGLE_PLAY_STORE*/ {
        .request_refund_support_url = string8_literal("https://support.google.com/googleplay/workflow/9813244"),
        .subscription_page_url      = string8_literal("https://play.google.com/store/account/subscriptions?package=network.loki.messenger"),
    },
    /*SESSION_PRO_PAYMENT_PROVIDER_IOS_APP_STORE*/ {
        .request_refund_support_url = string8_literal("https://support.apple.com/118223"),
        .subscription_page_url      = string8_literal("https://account.apple.com/account/manage/section/subscriptions")
    }
};

The general usage pattern for the request and response code available in pro_backend.{hpp,h}

GetProProofRequest  request       = {...};                                           // Build the request object
std::string         request_json  = request.to_json();                               // Convert it to json
std::string         response_json = send_net_request(pro_backend_url, request_json); // Send POST request using your networking APIs
GetProProofResponse response      = GetProProofResponse::parse(response_json);

if (response.status == SESSION_PRO_BACKEND_SUCCESS_STATUS) {
  // Success! Use the values in the struct
} else {
  // Failure! Parsing errors or input validation errors. Errors are held in the errors array
  for (auto it : response.errors)
    fprintf(stderr, "ERROR: %s\n", it.c_str());
}