Add support for using manual cooldowns on single commands #1166

Job	Run time
Build docs	1m 22s
Rustfmt	14s
Clippy	51s
Test (MSRV)	1m 20s
Test (stable)	2m 0s
Test (beta)	2m 3s
Test (nightly)	1m 47s
Test (macOS)	1m 26s
Test (Windows)	4m 31s
Test (no features)	1m 12s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 13s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 19s
	19m 18s

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