Add support for --public-network-access for az functionapp config namespace

There seems to be no way of setting the publicNetworkAccess attribute value (Enabled or Disabled) of the functionapp namespace, neither during creation, nor update (with az functionapp config create|update).

This means that, although we can automate the creation of Function App Inbound Private Endpoints and Function App VNet Integrations, we must go to each Function App and manually disable Public Access to our Function Apps.

**Describe the solution you'd like**
I'd simply like/expect to be able to run any of the following commands:

- az functionapp config --name myFA --resource-group myRG --public-network-access enabled
- az functionapp config --name myFA --resource-group myRG --public-network-access disabled

- OR -

- az functionapp config --ids myFAID --resource-group myRG --public-network-access enabled
- az functionapp config --ids myFAID --resource-group myRG --public-network-access disabled

**Describe alternatives you've considered**
It doesn't exist under "az webapp config set" either.

**Additional context**
Without this, everyone who wishes to create or manage secure Function Applications must go directly to each resource and toggle this attribute, manually, while the rest of the same page can be updated via the Azure CLI: