Add ability to make RoleAssignments directly against the Resource being created

[rudiv]

When creating a RoleAssignment, a separate Bicep Resource file is created that has an existing resource to scope against, as part of the overall rethink of the API to make it more "production-ready", think specifically about this.

Something such as a follow-on from the base builder would work great.

var id = builder.AddManagedIdentity("myidentity");
var maps = builder.AddAzureMapsAccount("maps")
    .AddRoleAssignment(id, MapsRoles.DataReader);

This could then add the RoleAssignment to the same file as the resource being created and avoid the need for duplication.