RUM-984 feat: Add sample rate for WebView Log events

[ncreated]

What and why?

📦 This PR adds logsSampleRate option to DatadogWebViewTracking. Now, it is possible to set custom sample rate that will be applied to logs received from Browser SDK that runs in a WebView.

Counterpart of Android's DataDog/dd-sdk-android#1629

     public static func enable(
         webView: WKWebView,
         hosts: Set<String> = [],
+        logsSampleRate: Float = 100,
         in core: DatadogCoreProtocol = CoreRegistry.default
     ) {

How?

• New parameter (logsSampleRate) is added in DatadogWebViewTracking public API.
• Log events are sampled right before forwarding them to DatadogLogs.

🎁 As an extra task, I enhanced tests for MessageEmitter by separating two concerns:

• parsing the message - now moved to WebViewMessageTests,
• forwarding the message to RUM or Logs - tested in MessageEmitterTests.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes

Custom CI job configuration (optional)

• Run unit tests for Core, RUM, Trace, Logs, CR and WVT
• Run unit tests for Session Replay
• Run integration tests
• Run smoke tests
• Run tests for tools/

[datadog-datadog-prod-us1]

Datadog Report

Branch report: ncreated/RUM-984/sample-webview-log-events
Commit report: c3f2314

✅ dd-sdk-ios: 0 Failed, 0 New Flaky, 2620 Passed, 0 Skipped, 12m 1.65s Wall Time

[maxep]

nit 👌