feat: add `envbuilder install-feature` command

I am looking into adding a module to the registry that would let template administrators install Dev Container features inside Coder Workspaces (even outside of `envbuilder`).

I think `envbuilder` already has the logic to install features, so I think the most logical thing to do would be to create a separate command just to install a given feature.

While Dev Container features are only shell scripts, there is still some logic needed for arguments in the manifest, e.g `dependsOn`.

See https://github.com/coder/modules/issues/307 for more context.

## Usage

`envbuilder install-feature `
Ideally, `source` could be either a Git URL, a local folder or a path to an image.

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cc: @johnstcn