feat: Add `STACK` and `DEMO` templating parameters #500

Job	Run time
General Pull Request Checks / Run Cargo Deny (bans licenses sources)	49s
General Pull Request Checks / Run RustDoc	7m 26s
General Pull Request Checks / Run Cargo Udeps	2m 48s
General Pull Request Checks / Run Cargo Deny (advisories)	46s
General Pull Request Checks / Run Cargo Tests	1m 11s
General Pull Request Checks / All tests passed	3s
Build stackablectl for x86_64-unknown-linux-gnu	1m 3s
Build stackablectl for aarch64-unknown-linux-gnu	1m 5s
Build stackablectl for aarch64-apple-darwin	2m 4s
Build stackablectl for x86_64-apple-darwin	1m 28s
	18m 43s

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