Support for "justUseNext" function

**Is your feature request related to a problem? Please describe.**
Currently need to use PowerShell to get next available priority for NSG, Firewall etc rules and then inject this value into ARM template for deployment.

**Describe the solution you'd like**
Similar to [440](https://github.com/Azure/bicep/issues/440) - would be nice to have a bicep function to get next available priority, free IP in a range etc:

```
resource nsg 'Microsoft.Network/networkSecurityGroups@2016-09-01' = {
name: networkSecurityGroupName
location: location
properties: {
securityRules: [
{
name: 'default-allow-rdp'
properties: {
priority: 1000 <----- This can be a Bicep function
sourceAddressPrefix: '*'
protocol: 'Tcp'
destinationPortRange: '3389'
access: 'Allow'
direction: 'Inbound'
sourcePortRange: '*'
destinationAddressPrefix: '*'
}
}
]
}
}
```

Start by reviewing the linked issue 440 and the example NSG security rule in this issue. Clarify whether the request is limited to next available priorities or also covers free IPs and other ranges; done requires an agreed function design and behavior before implementation can be scoped.

Written by the indexing model from the issue text.