Avoid copy in `SentryEnvelopeItem.envelopeItemStream` #1358

Job	Run time
cancel-previous-workflow	3s
android \| ubuntu-latest \| stable	1m 4s
android \| ubuntu-latest \| beta	1m 0s
linux \| ubuntu-latest \| stable	1m 30s
linux \| ubuntu-latest \| beta	1m 31s
ios \| macos-latest \| stable	59s
macos \| macos-latest \| stable	1m 0s
windows \| windows-latest \| stable	2m 30s
analyze / cancel-previous-workflow	3s
analyze / analyze	1m 31s
analyze / package-analysis	1m 38s
	12m 49s

Provide feedback