Add `dnx` command to dotnetup for standalone tool execution

## Summary

Add a `dnx` command to `dotnetup` that enables running standalone .NET tools without requiring the full SDK. This provides a managed alternative to the standalone `dnx` script with additional benefits like auto-updates and signed binary acquisition.

## Background

We've previously discussed adding `dnx` (or similar functionality) even before `dotnetup` existed, primarily driven by the rise of AI-powered tools that we want to make more accessible to users. The standalone `dnx` script already exists as a single-file solution, but integrating it into `dotnetup` provides several advantages that a simple script cannot offer.

## Motivation

- **AI Tool Accessibility**: AI-powered developer tools are becoming increasingly common. Users should be able to run these tools easily without needing a full SDK installation.
- **Managed Updates**: Unlike a standalone script, `dotnetup` can automatically update `dnx` when new versions are available.
- **Signed Binaries**: We can ensure users always get a signed, verified version of `dnx` through `dotnetup`'s acquisition pipeline.
- **PATH Management**: `dotnetup` already handles PATH setup for other components; extending this to `dnx` provides a consistent experience.
- **No SDK Required**: Users can run .NET tools in environments where installing the full SDK isn't practical or desired.

## Proposed Functionality

### Commands

```bash
# use dnx (comes pre-bundled (?))
dotnetup dnx
# Run a tool via dnx (after installation)
dnx [args...]
```

Wondering if we need a muxer for any of this. (nuget.client apis are not aot'd for example.)