fix: Respect existing baggage header instead of overwriting it

[kevinrenskers]

📜 Description

SentryNetworkTracker adds a baggage header with some sentry keys and values, but it does so by completely overwriting any possibly existing baggage header. And when we remove the baggage header when SentrySDK.currentHub.scope.span is nil, we also do that by removing the entire baggage header instead of only the sentry-trace_id key that we actually need to remove.

Now we respect the existing baggage header, if it's there, by modifying it.

💡 Motivation and Context

Closes #1911

💚 How did you test it?

Unit tests.

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• Review from the native team if needed
• No breaking changes

🔮 Next steps

[kevinrenskers]

The only test that keeps failing is the "UI Tests on iOS 12 Simulator" one, with the usual "Failed to get matching snapshots: Timed out while evaluating UI query". But I haven't touched the UI in this PR at all, so that's weird. I'll download the iOS 12 simulator on my computer, see if I can spot the problem.

[kevinrenskers]

Sample app looks like this in iOS 12 🙈

Fixed it.

[kevinrenskers]

We have one failing UI test on iOS 12, which is unrelated to this PR and has started happening a while ago. I created a separate issue for this (#2000), I don't think it should block this PR.

[brustolin]

LGTM

[brustolin]

LGTM