refactor: simplify fspy::Command API and add pre_exec hook #52

Job	Run time
Test (ubuntu-latest, x86_64-unknown-linux-gnu)	1m 4s
Test (namespace-profile-mac-default, aarch64-apple-darwin)	45s
Lint	1m 20s
Test (windows-latest, x86_64-pc-windows-msvc)	4m 22s
done	-1s
	7m 30s

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