Avoid copy in `SentryEnvelopeItem.envelopeItemStream` #1348

Job	Run time
android \| ubuntu-latest \| stable	1m 7s
cancel-previous-workflow	7s
android \| ubuntu-latest \| beta	1m 1s
linux \| ubuntu-latest \| stable	1m 27s
linux \| ubuntu-latest \| beta	1m 27s
ios \| macos-latest \| stable	1s
macos \| macos-latest \| stable	1s
windows \| windows-latest \| stable	2m 7s
analyze / cancel-previous-workflow	2s
analyze / analyze	1m 35s
analyze / package-analysis	1m 36s
	10m 31s

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