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.

Look at the existing toolchain installation logic in `midenup`, likely in a command module like `src/install.rs`. Understand how `miden-toolchain.toml` is parsed and used. The goal is to add a new CLI command or flag to install partial toolchains interactively, possibly prompting for each component. Check for existing interactive prompt patterns in the codebase. 'Done' means the CLI can install partial toolchains with optional prompts, and the default `midenup install stable` behavior remains unchanged.

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