Define the stream cost dynamically

[SofiiaVarich]

Expected behavior

At the moment we read the stream costs from instants. This means regardless of the subscription price, all of the costs the same for the user, producing no incentive for the users to purchase bigger subscriptions.

Actual behavior

Subscriptions should define the stream cost based on their price. This should be defined dynamically at the time of subscription creation.
We can specify a discount in the subscription.create command so that the stream cost is defined as:

streamCost = BASE_COST * (1 - discount);

then use the streamCost in the audio.stream command.

subscription.consumable -= subscription. streamCost;
// ...
income: audio.owners[index].income + (subscription .streamCost * BigInt(owner.shares)) / BigInt(100),