fixes for issue #2423

[menswearUK]

changed ApplicationSecret variable from string to PSCredential in functions Start-M365DSCConfigurationExtract, Export-M365DSCConfiguration, and Get-M365DSCTenantDomain. Removed line converting the string to a pscredential as it is no longer required, added a line to convert back to string for final output

Pull Request (PR) description

Since The updates to require ApplicationSecret to be in the form of a PSCredential I have had errors when exporting previous configurations.
I think the cause is a few functions which are still expecting ApplicationSecret to be passed as a string. I've amended these functions to use the PSCredential. There are also a couple of changes where the new type needs to be accounted for. I've tested it exporting the Intune workload and the IntuneAppProtectionPolicyAndroid component with no errors,
I believe All the configurations were updated at the same time to require the ApplicationSecret to be a PSCredential but I haven't tested any other configurations

This Pull Request (PR) fixes the following issues

Fixes #2423

[NikCharlebois]

Changing this will have a major breaking impact on customers. We had taken the decision to minimize the impact of the ApplicationSecret DSC param to a minimum by keeping the export process as string. I am reviewing issue #2447 and will have to work on a fix that doesn't involve changing the Export-M365DSCConfiguration cmdlet to take AppSecret as PSCredential. Will update this PR once the new one is available.

[NikCharlebois]

Fixed with #2449