Revert `Request::params` back to `Option` #4805

Job	Run time
Test (nightly)	3m 12s
Build docs	40s
Test (macOS)	2m 55s
Check minimal versions	36s
Test (stable)	3m 14s
Test (beta)	2m 59s
Examples	3m 54s
Test (Windows)	4m 17s
MSRV	0s
Test (native-tls)	3m 2s
Test (no default features)	1m 2s
Test (no cache)	2m 54s
Test (no gateway)	2m 15s
Test (unstable Discord API features)	3m 4s
Test (simd-json)	3m 14s
Test (chrono, chrono)	56s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 10s
Test (builder without model, builder, true)	36s
Test (unstable Discord API (no default features), unstable_discord_api, true)	47s
	40m 47s

Provide feedback