xGroup: support adding IIS AppPoll accounts as members

[bozho]

Pull Request (PR) description

This is a quick attempt at adding support for IIS AppPool group members.

The approach I took is to use [System.Security.Principal.NTAccount] to resolve SID for local accounts for which scope name does not resolve to computer name, and use the SID to find the identity principal.

This Pull Request (PR) fixes the following issues

Task list

• [x] Added an entry to the change log under the Unreleased section of the file CHANGELOG.md. Entry should say what was changed and how that affects users (if applicable), and reference the issue being resolved (if applicable).
• [ ] Resource documentation added/updated in README.md.
• [ ] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [ ] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [ ] Unit tests added/updated. See DSC Community Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• [x] New/changed code adheres to DSC Community Style Guidelines.

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This change is