RUM-8627: Collect App Hitches in the SDK

[simaoseica-dd]

What and why?

This PR creates the engine to collect View Hitches (Slow frames) that occur during the lifecycle of a View.
The proposed solution is defined in the internal RFC.

How?

To scale the entities that observe the render loop, it was created the DisplayLinker.
Any reader (RenderLoopReader) that wants to be notified about frame rendering can register to the DisplayLinker:
displayLinker.register(vitalRefreshRateReader)

This PR lists two RenderLoopReader:

• VitalRefreshRateReader - A class that reads the refresh rate (frames per second)
• ViewHitchesReader - A class that reads View Hitches or Slow frames.

For a better understanding of the relationships, the figure below depicts the idea:

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)

[datadog-datadog-prod-us1]

Datadog Report

Branch report: simaoseica/RUM-8627/collect-app-hitches-in-the-sdk
Commit report: 4972032
Test service: dd-sdk-ios

✅ 0 Failed, 1304 Passed, 2522 Skipped, 1m 26.09s Total duration (1m 12.71s time saved)