Add description field for Ci::InstanceVariable and InheritedCiVariable

What does this MR do and why?

Additional backend implementation for adding description to CI variables. Implements #378938 (closed) and used in !140547 (merged).

This MR creates the following changes:

• Exposes description in variable_params_attributes from the admin/group/project controllers.
• Adds the optional description field to Ci::InstanceVariable, allowing users to add a description to their instance-level (admin) CI variables. This field is already available for group-level and project-level CI variables.
• Include description field in the query results for inherited CI variables and instance CI variables.

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Screenshots or screen recordings

Instance-level CI Variables	Inherited CI Variables

How to set up and validate locally

API:

1. Generate a Personal Access Token via http://gdk.test:3000/-/profile/personal_access_tokens.
2. Create a new instance variable with a description.

curl --request POST --header "PRIVATE-TOKEN: <PERSONAL_ACCESS_TOKEN>" "http://gdk.test:3000/api/v4/admin/ci/variables" --form "key=NEW_ADMIN_VARIABLE" --form "value=ADMIN_VAR_KEY" --form "description=This is a new instance variable."

From the UI:

You can also run !140547 (merged) to verify that you can add descriptions to instance-level variables from the UI.

GraphQL Queries (for instance-level CI variables and inherited CI variables)

# instance-level ci variables
query {
  ciVariables(first: 10) {
    nodes {
      id
      key
      value
      description
    }
  }
}

# inherited ci variables
query {
  project(fullPath: "kitchen-sink/bakery") {
    inheritedCiVariables(first: 10) {
      nodes {
        id
        key
        description
      }
    }    
  }
}

Related to #418331 (closed)