Create SubscriptionDataRequest

[Almightywesley]

Unable to Create SubscriptionDataRequest Outside Buckaroo SDK

Issue

Currently, it is not possible to instantiate SubscriptionDataRequest from outside the Buckaroo SDK, making it difficult for programmers to create subscriptions.

Proposed Solution

To enable subscription creation, adding the following method to the ConfiguredDataRequest class would be beneficial:

public SubscriptionDataRequest Subscription()
{
    return new SubscriptionDataRequest(this);
}

Expected Usage

With this change, users will be able to create subscriptions as follows:

var buckarooRequest  = buckarooClient.CreateRequest()
                .Authenticate(WebsiteKey, SecretKey, isLive, culture)
                .DataRequest()
                .SetBasicFields(new DataBase { OriginalTransactionKey = transactionId })
                .Subscription()
                .CreateSubscription(subscriptionCreateRequest);
var result = await buckarooRequest.ExecuteAsync();

This enhancement would align SubscriptionDataRequest with other request types in the SDK, improving usability and consistency.