Types system should be able to handle more complex type definitions

_Created out of discussion from #7840_

Take a case like `keyVault`:

```bicep
resource kv 'Microsoft.KeyVault/vaults@2021-11-01-preview' = {
...
properties: {
...
accessPolicies: [] // required if "enableRbacAuthorization" property is not provided
enableRbacAuthorization: true // required if "accessPolicies" is not provided
}
}
```

Neither `accessPolicies` nor `enableRbacAuthorization` are explicitly required, but one of them is required if the other is not included. We need a way to express this complexity in our type system and validate it.

Start by reviewing the discussion in #7840 and the keyVault example in this issue. Determine how the type system could express that either accessPolicies or enableRbacAuthorization must be provided, and define validation behavior for that conditional requirement.

Written by the indexing model from the issue text.