[Feature]: Update-AzFunctionAppSetting should support slot settings

[Francisco-Gamino]

Description of the new feature

Update-AzFunctionAppSetting does not support slots.

This issue was reported at #17309

Proposed implementation details (optional)

No response

[Francisco-Gamino]

/cc @AnatoliB @stefanushinardi @michaelpeng36

[ggailey777]

@Francisco-Gamino after we add this functionality, we should add PS examples for the slot migration stuff here: https://docs.microsoft.com/azure/azure-functions/functions-versions