[enhancement]: Add PowerShell Core 7 execution handler

[jessehouwing]

Describe your feature request here

All tasks that use the PowerShell and PowerShell3 execution handler rely on PowerShell 5.1 as the main execution engine of the task. Any task that relies on PowerShell Core must call PowerShell Core either from PowerShell 5.1 or from Node.

Back in 2015 when the agent was first introduced and PowerShell Core was mostly a gimmick, it made sense, maintaining 2 different but mostly equal execution handlers only add maintenance overhead.

Fast-forward 8 years and PowerShell Core is now the gold standard. It's faster, cross platform and most new features come to it, including fancy language features. Most of all its cross platform and it doesn't depend on 4MB of random node modules.

It's time to introduce a proper PowerShellCore execution handler that doesn't rely on PowerShell 5.1, that has its own VstsTaskSdk and that runs cross-platform.

Why?

• It's faster. In the past days I've been looking at hosted agent performance and pwsh: as well as AzureCLI and AzurePowerShell and PowerShell tasks configured to use PowerShell Core add a few seconds overhead to each task execution.
• It's cross platform. I'm no star in Bash nor a hero in Typescript, but I know my way around the CLR and have written quite a bit of PowerShell, let me take my skills to Linux and Mac as well.
• It's more secure than Node based Tasks. Node based tasks easily carry 100s of additional Node Packages to do things that are built-in to PowerShell Core. XML parsing? Json parsing? Zipping archives, XSLT, JsonPath, REST API calls, you name it. In my research I can't find any PowerShell based extension with security vulns, yet at the moment there is no Node based tasks that doesn't trigger at least 2 critical warnings.
• It's where the current state of the market is at. Even PowerShell itself tries to tell me every time I accidentally open the wrong shell: Install the latest PowerShell for new features and improvements! https://aka.ms/PSWindows.
• GitHub Actions already has support for PowerShell core as default script handler and you can use PowerShell Core in composite actions to build your tasks. Having the same option in Azure Pipelines would make it easier to port tasks between the wo.

[ckrueger1979]

Dublicate of #4386

[kirill-ivlev]

Closing as duplicate.