[CHNL-16564] update script and message handling

[ab1470]

Description

This PR makes some updates to the KlaviyoWebViewModeling protocol, all conforming ViewModels, and the KlaviyoWebViewController. We will need these changes in order to handle the javascript messages that the in-app forms emit.

With this PR, I'm decoupling the javascript that gets injected into the WKWebView from the message handlers that get injected. This will allow us, in a future PR, to tell the WKWebView to listen for messages from the KlaviyoNativeBridge, even though we aren't injecting a script with the same name.

Check List

• Are you changing anything with the public API?
• Have you tested this change on real device?
• Are your changes backwards compatible with previous SDK Versions?
• Have you added unit test coverage for your changes?
• Have you verified that your changes are compatible with all the operating system version this SDK currently supports?

Manual Test Plan

1. I've tested this using Xcode previews and with the iOS test app

[ajaysubra]

Some minor questions but looks good to me