Look up existing resources by properties other than name

**Is your feature request related to a problem? Please describe.**
If one attempts to deploy a role assignment that already exists, the deployment will fail instead of gracefully acknowledging that the resource already exists and skipping it. If there were an "exists" method as covered in [this issue](https://github.com/Azure/bicep/issues/4023), I would still have a hard time applying it to role assignments.

A role assignment has the following properties:
- RoleAssignmentId
- Scope
- DisplayName
- SignInName
- RoleDefinitionName
- RoleDefinitionId
- ObjectId
- ObjectType
- CanDelegate
- Description
- ConditionVersion
- Condition

Unfortunately, none of those are the `name` property that must be used to describe an existing resource in Bicep.

**Describe the solution you'd like**
I'd like to have some way to describe such an assignment nonetheless using any of the other properties, as in the following:

```
resource assignment 'Microsoft.Authorization/roleAssignments@' existing = {
DisplayName: 'MyName'
RoleDefinitionName: 'MyRoleName'
}
```

Of course, this is no use until there is an `exists` method available, but following that, it would be an ideal to have it on the roadmap afterwards.

Start with the linked issue 4023 and the role-assignment properties listed here; investigate how existing-resource declarations currently require a name and how an exists method would interact with property-based lookup. Done means a defined and implemented way to identify existing role assignments by a non-name property, with its behavior documented and tested.

Written by the indexing model from the issue text.