Impossible to create certain sub-resources due to name restrictions

[peter-bertok]

Bicep version
Bicep CLI version 0.2.59 (5b0314c)

Describe the bug
I tried to update permissions for a Key Vault incrementally using a snippet such as the following:

resource webapp_kv_access 'Microsoft.KeyVault/vaults/accessPolicies@2020-04-01-preview' = {
  name:'${prefix}/add'
  properties:{
    accessPolicies:[
      {
              
      }
    ]
  }
}

Additional context
The error is BCP036, saying that the name must be "add|remove|replace", but this is a sub-resource, which means that it must have a prefix. This naming restriction is only valid when the subresource is defined inline, and that's actually a useless scenario for this particular resource. It is intended to be defined using the full resource syntax to enable incremental updates. In this case, I'm trying to have a bunch of App Service web apps all have access to the same KV.

It looks like BICEP is getting the restriction from here: https://github.com/Azure/azure-resource-manager-schemas/blob/54b5ec6b33f2f21d9968140ab67c61e6eab39fba/schemas/2016-10-01/Microsoft.KeyVault.json#L84

My suggestion is that resource name restrictions based on string enums should only consider the part after the last forward slash. If that's impossible (due to dynamic name generation), then it should simply ignore name restrictions for sub-resources.

[alex-frankel]

Closing as dup of #836