Support web_categories for azurerm_firewall_policy_rule_collection_group

[jleonelion]

This request to enhance azurerm_firewall_policy_rule_collection_group so support argument web_categories since this is an essential feature of Azure Firewall when creating policies to control site access from Azure.

Something like this in the above referenced TF file

web_categories = try(rule.value.web_categories, null)

Assign to me if you'd like.

[jleonelion]

@arnaudlh @LaurentLesle I have changes for this queued up. Please assign and let me know the branch you would like me to submit PR to.