[5.3] Streamline update purchasable stock level

[nfourtythree]

Description

To help when developers are programmatically creating variants or updating stock on a variant, this PR adds some helpful service methods.

WIth this, there is now a more streamlined way to update a variant's stock.

$variant = Variant::find()->one();
$quantity = 10;

// Simple call
Plugin::getInstance()->getInventory()->updatePurchasableInventoryLevel($variant, $quantity);

// If you want to `adjust` rather than `set` you can do that as follows
Plugin::getInstance()->getInventory()->updatePurchasableInventoryLevel($variant, $quantity, ['updateAction' => InventoryUpdateQuantityType::ADJUST]);

[linear]

COM-271 Improve purchasable stock updating