Support optional object type attributes in Nomad HCL variables

### Proposal

Support `optional(...)` in object type attributes, similar to the [equivalent feature from Terraform](https://developer.hashicorp.com/terraform/language/expressions/type-constraints#optional-object-type-attributes)

### Use-cases

Variables that use complex types (e.g. `list(object({ ... }))`, where some fields are optional, currently have to be specified in full, or have to be defined as maps where the burden of type validation falls on the template author.

### Attempted Solutions

* Using a `type = map(string)` and checking each field for existence/type-correctness
* Using a `type = string`, setting the value to some structured data format, like JSON or YAML, which is then `(yaml|json)decode`d and its contents' fields checked for existence/type-correctness