Add isInitialized and stopInstance methods to ObjC API

[0xnm]

What and why?

This PR adds isInitialized and stopInstance methods to ObjC API which were missing, but exist in Swift API.

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: nogorodnikov/add-stopinstance-and-isinitialized-to-objc-api
Commit report: 169d46f
Test service: dd-sdk-ios

✅ 0 Failed, 2767 Passed, 0 Skipped, 5m 40.04s Wall Time
🔻 Test Sessions change in coverage: 8 decreased, 4 increased, 1 no change

🔻 Code Coverage Decreases vs Default Branch (8)

This report shows up to 5 code coverage decreases.

• test DatadogCoreTests tvOS 79.32% (-0.3%) - Details
• test DatadogLogsTests tvOS 45.09% (-0.28%) - Details
• test DatadogLogsTests iOS 45.04% (-0.28%) - Details
• test DatadogCrashReportingTests tvOS 27.72% (-0.18%) - Details
• test DatadogCrashReportingTests iOS 27.71% (-0.18%) - Details

[maxep]

👍

The base branch was changed.