Add `CreateAttachmentStream` type #864

Job	Run time
Test (macOS)	4m 55s
Test (nightly)	2m 59s
Build docs	49s
Examples	2m 42s
Check minimal versions	49s
MSRV	0s
Test (stable)	2m 36s
Test (beta)	2m 59s
Test (Windows)	4m 39s
Test (native-tls)	2m 31s
Test (no default features)	51s
Test (no cache)	2m 30s
Test (no gateway)	1m 50s
Test (unstable Discord API features)	2m 46s
Test (chrono, chrono)	55s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 6s
Test (builder without model, builder, true)	24s
Test (unstable Discord API (no default features), unstable_discord_api, true)	41s
	36m 2s

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