[Bug] Some commands change default console colors

[atbagga]

Describe the bug
Some azure-devops CLI extension commands change the default console colors, specifically those that issue the statement: "This command group is in preview...". The problem is that subsequent console output (i.e. regular informational text, for example produced by Write-Output) is completely unreadable (black on black).

To Reproduce
Azure Cli Version: 2.0.76
Use az --version
Look for something like-
azure-cli (2.0.70)

Azure-Devops extension version: 0.15.0
Use az --version
Look for something like-
Extensions:
azure-devops (0.15.0)

Steps to reproduce the behavior:

1. Open Visual Code integrated terminal for PowerShell Core 6
2. login using az login or az devops login
3. Run command "az boards iteration project list --depth 10 --org $orgUrl --project $projectName
4. See "This command group is in preview..." message (blue on black text)
5. Try typing another command in the integrated terminal (PowerShell Core) window, nothing is visible as the console colors for regular text are now black on black.

Expected behavior
The only way I've found to recover the proper console colors is to invoke the [Console]::ResetColor() method, but this is a hack and devops cli shouldn't be interfering with the Visual Code PS Core console colors in this way. Note that this only happens in the Visual Code (1.40.2) terminal/console window - the behavior is fine in a PowerShell Core 6 console window.

Screenshots
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Debug logs
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Additional context
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Reported by @skeeler here - Azure/azure-devops-cli-extension#907

[jiasli]

Completed in #181. Color can now be disabled.