Investigate tree shaking of transaction code when span streaming is used #22580

Span streaming adds a hefty bundle size increase (~5KB gzip, 2-3KB added via the spanStreamingIntegration, rest is in core code) to the SDK, when enabled. In v11, streaming becomes the default, so the size increase is always there. We should investigate if we can provide a tree shaking flag for bundle-size conscious users that takes care of removing transaction code paths that are "dead code" for streaming mode.

This is also a good exercise for us to determine which code paths we should remove once transactions are no longer supported by the SDK (v12?). Plus it will give us a projection of the since decrease this will yield.