Implement new variable attribute - required

[AustinAbro321]

Is your feature request related to a problem? Please describe.

I believe it would be a good idea to have an attribute on variables - required. This would be useful in cases where someone is deploying and doesn't have a variable that's required for the package. It's plausible a user may have an updated version of a package, and is still accidentally using an old config file for example.

Describe the solution you'd like

• Given I have a variable "EXAMPLE_VAR" with the attribute required: true
• When I run zarf package deploy --confirm without "EXAMPLE_VAR" being set
• Then I get an error and the package doesn't deploy

Describe alternatives you've considered

You can use the attribute with a simple regex pattern: ".+" to achieve the same affect however users may not think to do this.

[Noxsios]

Related to #2096