Install PowerShell7 resource

[SteveL-MSFT]

Summary of the new feature / enhancement

A resource that simply installs PowerShell7 (if not already installed, so test should be implemented probably). This solves some bootstrapping scenarios and only require dsc to then get PS7. This would be cross-platform abstracting install of msi/appx/rpm/deb/brew. On non-Windows, it would need to be a Bash script while on Windows, we could still rely on Windows PowerShell.

What we don't support is a way (without an exe) to call different scripts based on the OS. So we either need a small exe that just calls a script appropriate for the OS or add a capability in the resource manifest to do different commands based on the OS (seems like the latter would create more problems to handle many edge cases).

Proposed technical implementation details (optional)

No response