@onlyIfNotExists() should support a definable clause

Issue:
=====
@onlyIfNotExists() is good, but it appears to validate the resource existence only on the resource name (or Id). There are some cases where the uniqueness of the resource is not in its name/Id but in another property of the resource.

For instance, a private DNS zone supports only one vnet link to any given vnet. That uniqueness is not in the resource Name or Id, but rather in VirtualNetworkId property. @onlyIfNotExists() does not help when there is already a vnet link which was created using a name other than the one you are attempting to check before creating.

`
@onlyIfNotExists()
resource virtualNetworkLink 'Microsoft.Network/privateDnsZones/virtualNetworkLinks@2020-06-01' = {
parent: privateDnsZone
name: vnetName
location: 'global'
properties: {
registrationEnabled: false
virtualNetwork: {
id: vnetId
}
}
}
`

Desired behavior:
============
@onlyIfNotExists() should support specifying a property of the resource to check for existence, or any other valid bicep operation which returns a true/false.
E.g. @onlyIfNotExists('VirtualNetworkId'=='')

No files, tests, or entry points are identified in the issue. Start by locating the implementation and tests for @onlyIfNotExists(), then clarify with maintainers which resource properties or boolean operations are supported and how the predicate should behave before changing code.

Written by the indexing model from the issue text.