RUM-1923 feat: add tracestate headers when using W3C tracecontext

[ganeshnj]

What and why?

Currently, we don't send vendor specific information (Datadog here) when using W3C tracecontext headers for distibuted tracing.

How?

This PR adds the vendor specific information to the tracecontext headers.

Datadog	tracecontext (W3C)
x-datadog-sampling-priority	s:{sampling-priority}
x-datadog-origin	o:{origin}

Example

x-datadog-trace-id: 1229782938247303441 (0x1111111111111111)
x-datadog-parent-id: 2459565876494606882 (0x2222222222222222)
x-datadog-sampling-priority: 2
x-datadog-origin: rum
x-datadog-tags: _dd.p.dm=-4,_dd.p.usr.id=baz64==

translates to

traceparent: 00-00000000000000001111111111111111-2222222222222222-01
tracestate: dd=s:2;o:rum;t.dm:-4;t.usr.id:baz64~~,othervendor=t61rcWkgMzE

x-datadog-tags is not supported by the SDK, hence it is not added to the tracecontext headers.

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: ganeshnj/feat/RUM-1923-tracestate
Commit report: 8041ac5

✅ dd-sdk-ios: 0 Failed, 0 New Flaky, 2652 Passed, 0 Skipped, 13m 16.21s Wall Time

[maciejburda]

LGTM