Resource type in resource definition should accept parameter or variable

**Is your feature request related to a problem? Please describe.**
When I define new resource I must use static string for resource type. E.g.

```bicep
resource storageAccount 'Microsoft.Storage/storageAccounts@2019-06-01' existing = {
name: resourceName
}
```

**Describe the solution you'd like**
Instead I'd like to do something like this

```bicep
resource storageAccount storageType existing = {
name: resourceName
}
```

where storageType is parameter or variable.

Start by reviewing how Bicep resource definitions handle the resource type expression in the examples shown. Determine whether parameter or variable values can be accepted while preserving the existing static resource-type syntax; done means the requested example compiles and existing-resource behavior remains valid.

Written by the indexing model from the issue text.