RUM-11578 Add Datadog context sharing in obj-c

[maciejburda]

What and why?

A short description of what changes this PR introduces and why.

How?

A brief description of implementation details of this PR.

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)