Add Support for Block Labels when using mapstructure-to-hcl2

[aidan-mundy]

• Adds support for Block Labels to mapstructure-to-hcl
  • Any string (or string pointer) field on a struct that also has the "label" option on the `hcl struct tag will be treated as a label, and generated schema include that field as a BlockLabel instead of an Attribute.
    • Example struct tag:'mapstructure:"nameLabel" hcl:",label"' (except with "`" instead of "'")
  • Configuring label field on the root-level Config struct for a component will trigger a crash before the build begins.
    • This happens because labels at that level are managed by another component of Packer. I consider this to be expected behavior because a label is not valid in that location.
• Updates HCL schema generation to actually use the required:"true" struct tag. Fields including that struct tag that are parsed as Attributes will have Required: true in their AttributeSpec.
  • This is a "breaking change" because the struct tag may be present in some cases where the field is not actually required. It is only used for documentation generation at the moment, so the fix for most users is to remove the label in cases where the field is not actually required.

[JenGoldstrich]

This is looking good to me Aidan! I think this approach makes sense to me, I think we should add testing for this in the existing units, like we talked about in slack I think it'd be good to have an example on your provisioners you're working on since that'd make manual testing of this super easy

We'll likely also want to update https://github.com/hashicorp/packer-plugin-scaffolding the scaffolding to have an example of this, but thats after this is released of course