feat(bench): implement random message and batch size for `iggy-bench` #1626

Job	Run time
check / cargo check	3m 18s
check / commit messages	3s
check / cargo clippy	3m 40s
check / cargo test docs	4m 8s
check / cargo fmt	13s
check / cargo sort	33s
check / cargo machete	1m 11s
build and test Linux-x86_64-musl	1m 36s
build and test macOS-aarch64	1m 40s
build Windows	1m 24s
Code coverage	0s
Backwards compatibility / check_commit_message	4s
Backwards compatibility / build_and_test	1m 25s
finalize_pr	1s
	19m 16s

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