Problem

PowerShell commands in the FinOps toolkit were showing a warning about upcoming breaking changes in the Get-AzAccessToken cmdlet:

WARNING: Upcoming breaking changes in the cmdlet 'Get-AzAccessToken':
The Token property of the output type will be changed from String to SecureString. Add the [-AsSecureString] switch to avoid the impact of this upcoming breaking change.

• The change is expected to take effect in Az version: '13.0.0'
• The change is expected to take effect in Az.Accounts version: '4.0.0'

The Start-FinOpsCostExport command was particularly affected since it makes multiple calls that triggered this warning.

Changes

This PR addresses the warning by modifying the PowerShell module's implementation:

1. Updated the Invoke-Rest.ps1 private function to correctly handle the upcoming changes to Get-AzAccessToken
2. Removed the .Token property access from the SecureString returned by Get-AzAccessToken -AsSecureString
3. Now directly converts the SecureString token to plain text without accessing the property that will be changing

The fix ensures compatibility with both current and future versions of the Az PowerShell module.

Fixes #987.

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