RUM-5841 feat(view-loading-api): implement addViewLoadingTime API

[ganeshnj]

What and why?

https://datadoghq.atlassian.net/wiki/spaces/RUMP/pages/3951001790/RFC+View+loading+time+manual+API

How?

This PR follows the RFC and introduces addViewLoadingTime API. The implementation follows the existing command pattern and updates the RUMViewEvent.view.loading when called which also triggers the view update.

This PR doesn't introduce any telemetry which will be added in upcoming PRs. We will require https://github.com/DataDog/dd-sdk-ios/pull/2019 to merged as prerequisite.

Review checklist

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

[datadog-datadog-prod-us1[bot]]

Datadog Report

Branch report: ganeshnj/feat/view-loading-api Commit report: a6f034a Test service: dd-sdk-ios

:white_check_mark: 0 Failed, 1869 Passed, 0 Skipped, 1m 15.03s Total Time :small_red_triangle_down: Test Sessions change in coverage: 3 decreased, 4 increased, 1 no change

:small_red_triangle_down: Code Coverage Decreases vs Default Branch (3)

• test DatadogTraceTests iOS 54.33% (-0.02%) - Details
• test DatadogCoreTests iOS 73.14% (-0.01%) - Details
• test DatadogSessionReplayTests iOS 30.49% (-0.01%) - Details

[shagedorn]

Since this was linked in the latest release: Is there any public documentation on how to use this feature?