[Functions] Need to update code that uses Get-AzAccessToken to accommodate breaking change

[Francisco-Gamino]

Description

Currently, we use (Get-AzAccessToken).Token to authenticate when pulling down the function app definitions from the Stacks API. This code needs to be updated because, after the release of Az 13, the return value will always be a secure string.

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[Francisco-Gamino]

/cc @amamounelsayed @AnatoliB @andystaples