Summary of the new feature/enhancement

As a PowerShell user, I want [ValidateSet] to support a dynamically generated sets from script blocks so that I can make terse, reusable sets of data, and my coworkers are provided useful IntelliSense from my work. In issue #3744, it was proposed to support a dynamically generated set for [ValidateSet]. An example was given with a script block, which would resolve the set. However, the actual merged PR laid the groundwork and implemented a variant, using classes and interfaces. I'd like this issue to specifically track the ability to use [ValidateSet] with a script block and receive IntelliSense feedback.

Proposed technical implementation details

For example:

[CmdletBinding()]
param (
    [ValidateSet({ (Get-MyGitRepos -Code -Config).master })]
    $Repo = "foo"
)