feat(bench): implement random message and batch size for `iggy-bench` #1636

Job	Run time
check / cargo clippy	3m 42s
check / cargo test docs	4m 11s
check / cargo check	3m 30s
check / commit messages	4s
check / cargo fmt	13s
check / cargo sort	32s
check / cargo machete	49s
build Windows	2m 49s
build and test macOS-aarch64	2m 42s
build and test Linux-x86_64-musl	4m 31s
Code coverage	0s
Backwards compatibility / check_commit_message	4s
Backwards compatibility / build_and_test	2m 27s
finalize_pr	1s
	25m 35s

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