xRDCertificateConfiguration: New resource to enable certificate management

[danielboth]

Pull Request (PR) description

New resource to enable the management of the certificates in a remote desktop deployment. This resource enables the configuration of each role individually, so users can have different certificates between roles.

This Pull Request (PR) fixes the following issues

• Fixes #63

Task list

• [X] Added an entry under the Unreleased section of the change log in the README.md. Entry should say what was changed, and how that affects users (if applicable).
• [X] Resource documentation added/updated in README.md.
• [X] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [X] Comment-based help added/updated.
• [X] Localization strings added/updated in all localization files as appropriate.
• [X] Examples appropriately added/updated.
• [X] 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