Add `traceparent` header support for .NET

[markushi]

This allows trace propagation to work when sentry-client SDKs communicate with otel instrumented backends.

Develop Docs:
https://develop.sentry.dev/sdk/telemetry/traces/#propagatetraceparent
https://develop.sentry.dev/sdk/telemetry/traces/distributed-tracing/#w3c-trace-context-header

[linear]

NET-442 Add `traceparent` header support for .NET