Implement tracing headers

[marandaneto]

Summary

Implement canonical PostHog tracing headers behavior for the Unity SDK.

Expected behavior

When tracing headers are enabled for an allowlisted host, outgoing requests include:

• X-POSTHOG-DISTINCT-ID
• X-POSTHOG-SESSION-ID

The SDK should not inject these headers for non-allowlisted hosts.

Spec

https://github.com/PostHog/sdk-specs/blob/main/acceptance/private/tracing-headers.feature

Notes

This should take into account Unity networking APIs and supported platform constraints.