Add optional navigation delegate to ios PlatformWebViewParams

[cyborch]

This PR adds the ability to set a navigation delegate on iOS

The in delegate must implement WKNavigationDelegateProtocol and is added to the iOS version of PlatformWebViewParams.

[Copilot]

Pull request overview

Adds support for providing a custom iOS WKNavigationDelegate-compatible delegate via PlatformWebViewParams, and forwards selected navigation callbacks from the library’s internal delegate to the user-provided one.

Changes:

• iOS: introduce PlatformWebViewParams(delegate: WKNavigationDelegateProtocol?) and plumb it through ActualWebView into the iOS WebView implementation.
• iOS: forward several WKNavigationDelegateProtocol callbacks (start, redirect, commit, finish, failures, process termination, decidePolicy) to the optional user delegate when implemented.
• Android: raises android.minSdk from 21 to 24 (appears unrelated to the PR’s stated purpose).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
webview/src/iosMain/kotlin/com/multiplatform/webview/web/WKWebViewObserver.kt	Adds an explicit final modifier (no functional change).
webview/src/iosMain/kotlin/com/multiplatform/webview/web/WKNavigationDelegate.kt	Forwards navigation delegate callbacks to an optional user-provided delegate from PlatformWebViewParams.
webview/src/iosMain/kotlin/com/multiplatform/webview/web/WebView.ios.kt	Adds iOS PlatformWebViewParams(delegate) and passes it into the internal navigation delegate.
webview/src/iosMain/kotlin/com/multiplatform/webview/jsbridge/WKJsMessageHandler.kt	Adds an explicit final modifier (no functional change).
webview/src/commonMain/kotlin/com/multiplatform/webview/web/WebView.kt	Updates KDoc to reflect iOS params usage (needs a type-name correction).
gradle.properties	Raises Android minSdk to 24 (breaking change; likely out of scope for this PR).

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