RUM-11470: Report Time to full display

[simaoseica-dd]

What and why?

This PR introduces a new app launch metric: Time to Full Display (TTFD).
It follows the proposal outlined in the internal RFC.

How?

It introduces a new public interface to report that the app is fully interactive:

public protocol RUMMonitorProtocol: AnyObject {
    /// Records the time to full display (TTFD) of the current app launch.
    func reportAppFullyDisplayed()
}

The TTFD logic is centralized in the RUMAppLaunchManager that:

• Reports the TTFD through an "app_launch" RUMVitalEvent.
• Decorates the TTFD with the startup type (cold or warm) and isPrewarmed value.
• Ensures the metric is reported only once and only for user-triggered launches.

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
• Add Objective-C interface for public APIs (see our guidelines (internal) and run make api-surface)

[mariedm]

Some minor comments, otherwise looks great!

[maxep]

Looks great! I left a minor question.

I have a larger suggestion (maybe it's planned for further PR): The TTID and TTDF would deserve coverage in IntergrationTests. Both in Units, using @ncreated's framework to build different app launch scenarios, and in UI Tests which will start an actual host app. Is this planned?

[simaoseica-dd]

@maxep
You can find the integration tests for the different scenarios here.
The UI Tests also have a task to be handled separately.