Investigate tree shaking of transaction code when span streaming is used

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.

Start by examining the spanStreamingIntegration and the core transaction code paths responsible for the reported bundle-size increase. Measure the SDK bundle with streaming enabled, identify transaction paths that can be removed through tree shaking, and compare the resulting gzip size. Done means a documented projection of the reduction and a clear proposal for the tree-shaking flag and future transaction cleanup.

Written by the indexing model from the issue text.