[Free Trials] Implement `siteHasCurrentInAppPurchases` check for web purchase flow

[iamgabrielma]

From: #10321 (comment)

It might be worth giving the real answer here, i.e. checking for the SiteID in this user's transactions. Or to error in some way – the site might have an In-App Purchase, even if you shouldn't be able to get to the view now. If the implementation is the same you could make it a protocol extension.