Optional attributes for "object" types in variables?

#### Community Note

Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request.
Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request.
If you are interested in working on this issue or have submitted a pull request, please leave a comment.

#### Description

I'd like to use "object" types for variables, but where some of the attributes are not required. I'd expect, when not specified, they would evaluate to the `default` value of the attribute. As a further example of the idea, Terraform has a similar concept, that they're [currently running as an experiment](https://www.terraform.io/language/expressions/type-constraints#experimental-optional-object-type-attributes).

#### Potential configuration

```
variable "example" {
type = object({
a = string # a required attribute
b = optional(string) # an optional attribute
})
default = {
a = "foo"
b = "bar"
}
}
```

#### Potential References