/plugin marketplace add should prefer shallow sparse checkout from pluginRoot

### Describe the feature or problem you'd like to solve

Support repos containing stuff beyond just plugins

### Proposed solution

Searching through github.com - there is nontrivial amount of repos publishing plugins alongside of the non-plugin code/infra/etc: https://github.com/search?q=path%3A**%2Fmarketplace.json+plugins&type=code

Not to force publisher to separate those into separate repo nor to force user to checkout unrelated stuff to their machines - we can use the `pluginRoot` metadata to indicate and fetch only relevant plugins.

### Example prompts or workflows

[dotnet/skills ](https://github.com/dotnet/skills) repo has plugins in .\plugins folder. Everything else is not intended for plugins consumers, but for the plugin authors and maintainers

Observing the `pluginRoot` would help keeping all the maintainer relevant stuff in same repo, while not pushing it to customers machines

### Additional context

_No response_

Start at the `/plugin marketplace add` flow and trace how the `pluginRoot` metadata is read and how repositories are currently checked out. Investigate Git's shallow sparse-checkout behavior for fetching only the plugin subtree. Done means repositories with non-plugin content install the relevant plugins without checking out unrelated files.

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