Handle metadata, extra, addendum_data in wallet_purchase

[RichardDale]

It is a trivial exercise to handle the extra field in wallet_purchase by modifying wallet_purchase, as follows:

public function wallet_purchase($amount, $reference, $wallet, $currency = "AUD", $extra = null, $metadata = null, $addendum_data = null)

and by adding:
if (!is_null($extra)) $payload["extra"] = $extra;
if (!is_null($metadata)) $payload["metadata"] = $metadata;
if (!is_null($addendum_data)) $payload["addendum_data"] = $addendum_data;

and finish it off by updating the function documentation by adding the following line:
* @param array<string,string> $extra an assoc. array of extra params to merge into the request
* @param array<string,string> $metadata an assoc. array of metadata params to merge into the request
* @param array<string,string> $addendum_data an assoc. array of addendum_data params to merge into the request