feat: Add `STACK` and `DEMO` templating parameters #501

Job	Run time
General Pull Request Checks / Run Cargo Tests	1m 26s
General Pull Request Checks / Run Cargo Deny (bans licenses sources)	50s
General Pull Request Checks / Run RustDoc	6m 52s
General Pull Request Checks / Run Cargo Deny (advisories)	48s
General Pull Request Checks / Run Cargo Udeps	2m 50s
General Pull Request Checks / All tests passed	3s
Build stackablectl for aarch64-unknown-linux-gnu	1m 18s
Build stackablectl for x86_64-apple-darwin	1m 48s
Build stackablectl for x86_64-unknown-linux-gnu	1m 6s
Build stackablectl for aarch64-apple-darwin	2m 5s
	19m 6s

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