Add support for using manual cooldowns on single commands (#269) #1167

Job	Run time
Rustfmt	4s
Clippy	49s
Test (MSRV)	1m 26s
Build docs	2m 16s
Test (stable)	1m 51s
Test (beta)	2m 0s
Test (nightly)	1m 44s
Test (macOS)	1m 51s
Test (Windows)	4m 24s
Test (no features)	1m 13s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 22s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 21s
	20m 21s

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