Draft: Refactor CI::Config::Entry::Variables

What does this MR do and why?

Refactors Gitlab::Ci::Config::Entry::Variables into Variable

Screenshots or screen recordings

• Remove Entry::Variables in lieu of Entry::Variable and use strategies for determining if the variable is simple or complex
• Make the variables in Entry::Root be a ComposableHash and pass in the Entry::Variable class as a composable_class

With all options for creating a variable, which are:

`variable: 'value'

and

variable: 
   value: 'value'
   description: 'description'

and

variable: 
   value: 'value'

.

Validations:

.

.