Add message_snapshots on Message #5904

Job	Run time
Test (nightly)	2m 45s
Test (macOS)	2m 8s
Examples	3m 54s
Check minimal versions	39s
MSRV	52s
Build docs	46s
Test (stable)	2m 44s
Test (beta)	2m 56s
Test (Windows)	5m 36s
Test (native-tls)	2m 32s
Test (no default features)	53s
Test (no cache)	2m 21s
Test (no gateway)	1m 54s
Test (unstable Discord API features)	2m 55s
Test (simd-json)	2m 41s
Test (chrono, chrono)	53s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 13s
Test (builder without model, builder, true)	40s
Test (unstable Discord API (no default features), unstable_discord_api, true)	35s
	38m 57s

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