Update *-Metadata cmdlet and Test-HelpInfoUri

[sdwheeler]

PR Summary

• Before this PR, the *-Metadata cmdlets did a naive parse of the metadata to isolate the frontmatter metadata. This PR changes the code to use Get-Document to parse the markdown and return the body and the frontmatter as properties of an object.
• Adds the AsJson parameter to Get-Metadata so that you can get the metadata as a JSON object.
• Add the HelpInfoUri parameter to Test-HelpInfoUri so you can override the HelpInfoUri property of a module. This allows you to test a new HelpInfoUri before changing the module manifest.

PR Checklist

• Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• Summary: This PR's summary describes the scope and intent of the change.
• Contributor's Guide: I've read the contributors guide.
• Style: This PR adheres to the Microsoft Learn style guide for PowerShell.

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