feat: Allow IScopeObserver to observe trace

[bitsandfoxes]

The Problem

Currently, errors on different layers - i.e. Unity C# -> Java -> C/C++ are not connected via trace ID. The Java and the native SDK allow for Hybrid SDKs to SetTrace(SentryId traceId, SpanId parentSpanId). From the Unity SDK's point of view, there is no mechanism right now that allows the .NET SDK to automatically propagate the trace to the native layers.

Proposal

Since the trace is held on the scope's PropagationContext we can leverage the already existing TwP functionality. Additionally, we can use the IScopeObserver to "observe" the relevant TraceID and ParentSpanID.

Allow the Unity SDK to SetTrace

There are multiple ways I can think of achieving this:

1. Make PropagationContext public
2. Add an accessors to the Scope to update the `PropagationContext
3. Add an InternalSentrySdk class where we can put SetTrace (and other things that regular users are not supposed to go). We have similar solutions like PrivateSentrySDKOnly for Cocoa and InternalSentrySdk in Java.
4. Make InternalsVisible to Sentry.Unity.

Update the IScopeObserver

This IScopeObserver can then be implemented by the respective native scope observer on the Unity SDK.
getsentry/sentry-unity#1997
This will also work for Android and iOS when building MAUI apps since. We'll just have to bump the native SDKs there.

Result

Unity SDK

The end result looks like this

.NET SDK

This also works on platforms where the .NET SDK brings support via sentry-native.
Since the SDK also has observers for iOS and Android, once those SDKs are updated, it'll work there as well.

Context

Relates to getsentry/sentry-unity#1997
Relevant change to sentry-java: getsentry/sentry-java#4137
Relevant change to sentry-native: getsentry/sentry-native#1137

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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[bruno-garcia]

I'm approving to unblock but give @jamescrosswell and @aritchie a chance to review this if they have time in the next day or two unless you need this out there asap

[bruno-garcia]

Do we need to follow up later with this once this is added to the Java and Apple SDKs?
If so ideally we create a ticket so folks here can add it later

[bitsandfoxes]

Tracking in #4074