Add custom priority to NSG rules

Currently the priority of network security rules is as follows are done in the order that the map is merged. Each rule will have a priority of 100 plus its index in `base_security_rules`

https://github.com/Azure/relational-infrastructure/blob/a29a2b431e0d689adcfdbf83ecf7b13f7d30e7fd/subscription_infra_map/locals.nsgs.tf#L256-L261

This ordering will generally be ok because the merging for base_security_rules goes:

allow_in_security_rules > allow_out_security_rules > deny_in_security_rules > deny_out_security_rules

We should allow for custom priorities. We could have each rule have a custom priority and custom "base" priority for both inbound or outbound. Say if you wanted the rules to start at 200 instead of the current hardcoded 100.