Retain webview cache on cancel

[kiftio]

What are you trying to accomplish?

When you close the checkout sheet, the preloading cache is cleared. This has been raised a few times as a bug. The core of the issue is we don't not know when the users cart becomes invalid

We need to avoid this pitfall:

• The merchant is not calling preload on cart change
• The user opens and closes checkout
• User modifies cart
• User opens checkout and is shown a stale checkout

The assumption in this PR is, if the merchant calls preload once, they are taking responsibility to call it again when cart updates

Ideally we want the best of both worlds:

• an up-to-date checkout (obv)
• a fast experience for when users just want to temporarily navigate away from checkout (eg to check another product) but doesn't modify the cart

---

[!Note] The current implementation is a bit different to the swift one. We're not tracking if clients have called preload. We're instead updating the cache, so that if preload isn't called, there shouldn't be anything in the cache to retain. (We do always add something to the cache, but with a TTL of 0, so it'll be thrown away as stale before presenting).

Test scenarios:

1. preloading enabled, preload() never called (add to cart, open checkout, close checkout, modify cart, open checkout)
2. preloading enabled, preload() called on each cart modification
3. preloading disabled
4. onRenderProcessGone
5. server error

Before you deploy

• [x] I have added tests to support my implementation
• [x] I have read and agree with the contributing documentation
• [x] I have read and agree with the code of conduct documentation
• [x] I have updated any documentation related to these changes.
• [x] I have updated the README (if applicable).

[kiftio]

Are we clearing the cache on error in this lib?

Yeah, we have a handleError function in CheckoutWebView that eventually either dismisses the dialog (which clears the cache), or just clears the cache if the dialog hasn't been displayed yet.