Take 3, final version.

PR Summary

This PR allows Get-Item -Stream, Get-Content -Stream, Clear-Content -Stream, Set-Content -Stream, Add-Content -Stream, and Remove-Item -Stream to see and address alternate data streams on directories, not merely on files.

Fixes #10570. Fixes #13656.

Supersedes #13650 (squashes all intermediary commits).
Supersedes #13795 (apparently some pull I did wanted to update the workflow, and my OAUTH token doesn't have workflow scope, and I can't figure out how to fix it)

Addresses addtional comments in #13795 regarding $PSIsContainer should be false on alternate data streams on directories because ADSes cannot contain other items.

PR Context

Issue #10570 has been open for a year. NTFS supports what are called "Alternate Data Streams" on both files and directories (multiple named discrete blobs of data which are associated with a single directory entry). PowerShell currently supports enumeration of these Alternate Data Streams on files, using the '-Stream' parameter to 'Get-Item'. It also supports manipulation of these alternate data streams on files, using the '-Stream' parameter to Set-Content, Add-Content, Clear-Content, and Remove-Item.

Unfortunately, the initial implementation of PowerShell only supported alternate data streams on files, not on directories. This makes an entire facility of the OS's file system invisible, and if an administration team is relying on PowerShell it makes an attractive place for a red team to store data to exfiltrate. (This is not an invitation to destroy the capability to store alternate data streams on directories, as they are useful for many purposes. It is merely a rationale for making their existence visible through PowerShell.)

To create and see an alternate data stream on a directory, use cmd.exe to run the following commands:

> mkdir 10570demo
> cd 1057demo
> echo "This is a file." > 10570demo.txt
> echo "This is an alternate data stream on the file." > 10570demo.txt:datastream
> mkdir bug10570
> echo "This is an alternate data stream on the directory." > bug10570:datastream
> dir /r

The output is something like:

D:\10570demo>dir /r
 Volume in drive D is DATA
 Volume Serial Number is 8FD3-BD69

 Directory of D:\10570demo

09/17/2020  02:59 PM    <DIR>          .
09/17/2020  02:59 PM    <DIR>          ..
09/17/2020  02:58 PM                20 10570demo.txt
                                    50 10570demo.txt:datastream:$DATA
09/17/2020  02:59 PM    <DIR>          bug10570
                                    55 bug10570:datastream:$DATA
               1 File(s)             20 bytes
               3 Dir(s)  88,185,401,344 bytes free

To see the failure of PowerShell being able to see the stream on the file, but not the directory:

> pwsh
PS > Get-Item *


    Directory: D:\10570demo

Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
da---           9/17/2020  2:59 PM                bug10570
-a---           9/17/2020  2:58 PM             20 10570demo.txt

PS > Get-Item * -stream *

PSPath        : Microsoft.PowerShell.Core\FileSystem::D:\10570demo\10570demo.txt::$DATA
PSParentPath  : Microsoft.PowerShell.Core\FileSystem::D:\10570demo
PSChildName   : 10570demo.txt::$DATA
PSDrive       : D
PSProvider    : Microsoft.PowerShell.Core\FileSystem
PSIsContainer : False
FileName      : D:\10570demo\10570demo.txt
Stream        : :$DATA
Length        : 20

PSPath        : Microsoft.PowerShell.Core\FileSystem::D:\10570demo\10570demo.txt:datastream
PSParentPath  : Microsoft.PowerShell.Core\FileSystem::D:\10570demo
PSChildName   : 10570demo.txt:datastream
PSDrive       : D
PSProvider    : Microsoft.PowerShell.Core\FileSystem
PSIsContainer : False
FileName      : D:\10570demo\10570demo.txt
Stream        : datastream
Length        : 50

PS > Get-Item bug10570 -stream *
PS >

Writing the tests revealed that Set-Content internally calls Clear-Content, which is hardcoded to not check for streams on directories. This was raised as an issue in Issue #13656, but I decided to put that into this PR as well, because it made it easier to write the tests.

PR Checklist

• PR has a meaningful title
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: or [ WIP ] to the beginning of the title (the WIP bot will keep its status check at Pending while the prefix is present) and remove the prefix when the PR is ready.
• Breaking changes
  • I'm assuming bucket 3: current behavior of Get-Item -stream when confronted with a directory was to silently do nothing, and this PR continues to do so in the event a directory doesn't have any alternate data streams. I would like someone to doublecheck, though.
• User-facing changes
  • Documentation needed
    • Issue filed: Get-Item -stream now works on directories. (PS #13941) MicrosoftDocs/PowerShell-Docs#6648
• Testing - New and feature
  • Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.