Add fuzzing setup for `Message::decode` #196

Job	Run time
Check spelling	1m 3s
Integration tests (ubuntu-latest)	49s
Check dependencies (ubuntu-latest)	3m 45s
Check formatting (ubuntu-latest)	1m 20s
Integration tests (macos-latest)	1m 57s
Check dependencies (macos-latest)	10m 25s
Clippy lints (ubuntu-latest)	1m 9s
Check formatting (macos-latest)	1m 51s
Integration tests (windows-latest, true)	2m 18s
Check dependencies (windows-latest)	8m 19s
Check formatting (windows-latest)	3m 10s
Clippy lints (macos-latest)	3m 0s
Clippy lints (windows-latest)	3m 15s
Unit tests (ubuntu-latest)	1m 25s
Unit tests (macos-latest)	1m 58s
Unit tests (windows-latest)	3m 45s
	49m 29s

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