Terraform-Like Optional Attribute Support

[sam-delap]

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Description

A similar optional() keyword to Terraform (Note: I know this has been brought up previously in #12182, there's just not a commmunity-visible place where this is being tracked to my knowledge).

Use Case(s)

Any use case where it would be useful to make a field within a Packer object optional to configure and provide per-field defaults

Potential configuration

variable "some-object" {
    type = object({
        some-field = optional(string, 'some-value')
        required-field = bool
    })
}

Potential References

https://developer.hashicorp.com/terraform/language/expressions/type-constraints#optional-object-type-attributes