feat: support async onShouldStartLoad requests

[kbulgakov-exo]

The existing approach with locking the main thread to make a synchronous call asking JS "can I proceed with this URL" isn't reliable. Even the recent timeout bump to 500ms #47 didn't help, we still experience the loading issues. Bumping the timeout even more (for example with #48) would inevitably lead to performance degradation because of the locked thread. We should come up with a more reliable solution

The bottleneck

I've put some logs to diagnose what exactly is the bottleneck. The JS callback works super fast, but the problem is that sometimes it seems to be called after Native already makes the decision after the 500ms default timeout:

 Timeline:
  0ms      Native dispatches event
           |
           |  (JS thread busy)
           |
  501ms    Native TIMEOUT → blocks navigation
           |
 >501ms   JS receives event, responds "allow"
           |
           ✗ Response ignored - decision already made 

The solution

Let's mirror the async architecture of the onShouldStartLoad calls from https://github.com/react-native-webview/react-native-webview. We can easily backport most of the code.

I recommend reviewing commit by commit

Notes:

• the first commit cherry-picks WebViewDecisionManager from upstream with minor formatting changes
• the other commits rely on the upstream code as well, but with a few minor changes to conform to our codebase. I’m going to leave the code references below.

Testing

• WebView loads content normally when JS responds with true
• WebView rejects loading when JS responds with false

[kbulgakov-exo]

Reference: https://github.com/react-native-webview/react-native-webview/blob/5bc526fce5b9d6225df183bdf3d8cf542007d90a/apple/RNCWebViewImpl.m#L1366-L1386

The allowing code is moved into the allowNavigation helper to be reused below to keep the old fallback behavior

[kbulgakov-exo]

Reference: https://github.com/react-native-webview/react-native-webview/blob/5bc526fce5b9d6225df183bdf3d8cf542007d90a/apple/RNCWebViewModule.mm#L21

[kbulgakov-exo]

tACK

• WebView loads content normally when JS responds with true
• WebView rejects loading when JS responds with false

[guten-exodus]

utACK nice

[kbulgakov-exo]

@guten-exodus Could you check whether this also addresses the pain points you had in Pay?