Helper function to get private link dns names

**Is your feature request related to a problem? Please describe.**
When working with private link, it can often be a pain to work out what the dns zone should be for private link. there is a table to map this at https://docs.microsoft.com/en-us/azure/private-link/private-endpoint-dns but it can be hard to find, and hard coding the zones also leads to problems if deploying to special regions

**Describe the solution you'd like**
It would be helpful if there was some kind of function that could take a resource or resource type, and output the private link zone for that resource

e.g. given an input of `Microsoft.Sql/servers`, it should return `privatelink.database.windows.net`

A few ideas of ways in which this could be implemented:
```bicep
resource sql 'Microsoft.Sql/servers@2014-04-01' existing = {
name: 'foo'
}

output idea1 = privateLinkDnsZone(sql.type)
output idea2 = privateLinkDnsZone(sql)
output idea3 = sql.privateLinkDnsZone()
```

Start with the Azure private endpoint DNS table linked in the issue and compare its resource-type and special-region mappings with the proposed inputs. The work is done when the supported input form and returned private-link DNS names are defined and verified for the stated Microsoft.Sql/servers example and relevant regional cases.

Written by the indexing model from the issue text.