[Baggage] Do not propagate incoming 3rd party baggage + Cleanup

[Lms24]

As discussed in our planning meeting on June 20, we will not continue to propagate incoming 3rd party baggage in the outgoing requests we instrument. Instead, we will:

On incoming requests:

• Only store sentry- values from the baggage header (if there is one) on the transaction

On outgoing requests:

• Append our sentry- values to an already existing baggage header
• Or add a baggage header to outgoing requests if there is no header yet

The reason for this change is that it is not our responsibility to do the job of another vendor. In fact, if we continue to propagate 3rd party baggage, we might even propagate something that our users do not want to be propagated (e.g. which is why they would e.g. not install that verndor's SDK in the service where a Sentry SDK is).

We will of course continue to propagate baggage that was already added by another SDK before us in the outgoing request (and, as outlined above, just append our values to that header). In return, we can only expect the other vendors' SDKs to follow the W3C spec and do the same.

This is a good opportunity to review our Baggage API and potentially cut down a few bytes of bundle size for logic and functions that are not needed anymore after this change. One mandatory cleanup task:

• move getBaggage() method from span to transaction (see ref(tracing): Move getBaggage() from Span to Transaction class #5299)