Support anonymous resources

[rjmholt]

There's no strong reason to require that all resources be named, since PSArm can take a resource definition and pass it around as a variable reference. With that in mind, PSArm should support anonymous resources where users can simply define an unnamed resource and pass it by variable reference rather than by name.

Related to https://github.com/PowerShell/PSArm/issues/32: when does a variable resolve to a name reference vs the full template value?