Change `CreateAttachment` to stream data from disk when possible #6100

Job	Run time
Test (macOS)	1m 59s
Test (nightly)	1m 21s
MSRV	0s
Build docs	42s
Check minimal versions	39s
Examples	2m 33s
Test (Windows)	4m 30s
Test (native-tls)	1m 54s
Test (stable)	1m 54s
Test (no default features)	50s
Test (beta)	1m 54s
Test (no gateway)	1m 30s
Test (zstd compression)	1m 54s
Test (unstable API + typesize, default unstable typesize, true)	57s
Test (chrono, chrono)	48s
Test (no cache)	1m 43s
Test (unstable Discord API features)	1m 55s
Test (zlib compression)	1m 59s
Test (zlib and zstd compression)	1m 51s
Test (builder without model, builder, true)	35s
Test (unstable Discord API (no default features), unstable, true)	31s
	31m 59s

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