provide buildkit like SDK for nix pure shell

[dr-housemd]

Buildkit being programmable is one of its major benefits.
Nix's build infra is way ahead than what Buildkit offers.

What we need is -

• An isolated sandbox environment with the dependencies mentioned in dev & runtime.
• Current working directory copied over.
• Execute commands sequentially/parallel and ability to control inputs & outputs.

This will allow us to-

• Build HCL build blocks for languages where vendoring isn't suitable
• Easily perform multi-arch builds with the native language toolchain
• Possibly, let users create their own interfaces in future.