feat: add partial toolchain support from CLI

Currently, installing partial toolchains is limited to the `miden-toolchain.toml` file for a specific directory.

This functionality could be re-surfaced in other areas of `midenup`.

@plafer suggested that the installation process could be interactive (i.e. prompts for verification before installing each component).

I believe the feature would mean an improvement in UX, however, maybe it could be limited to cases where `miden` auto installs toolchains instead of always prompting for confirmation; keeping `midenup install stable` the same.