We need a way to update site config setting for a function app slot

## Description of the new feature

There are a few setting that users need to be able to change in site settings, which are not accessible from the current cmdlets.

* **`linuxFxVersion`** - sometimes is used to pin app to a specific Linux image
* **`powerShellVersion`** - this controls the version of PowerShell used by the app.

There may be other site config settings of interest, but these are the two that are known and a PowerShell way must be documented, since there's no portal UX for this.

## Proposed implementation details (optional)

As suggested by @Francisco-Gamino:
There should be two new cmdlets:
* `Get-AzFunctionAppConfig`
* `Update-AzFunctionAppConfig`