feat: Add `STACK` and `DEMO` templating parameters #499

Job	Run time
General Pull Request Checks / Run Cargo Udeps	9m 9s
General Pull Request Checks / Run RustDoc	9m 57s
General Pull Request Checks / Run Cargo Deny (bans licenses sources)	49s
General Pull Request Checks / Run Cargo Deny (advisories)	50s
General Pull Request Checks / Run Cargo Tests	4m 22s
General Pull Request Checks / All tests passed	3s
Build stackablectl for x86_64-unknown-linux-gnu	4m 19s
Build stackablectl for aarch64-unknown-linux-gnu	3m 5s
Build stackablectl for aarch64-apple-darwin	5m 4s
Build stackablectl for x86_64-apple-darwin	5m 11s
	42m 49s

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