Add description optional field for Ci::Variable

Ref: #409641 (closed)

What does this MR do and why?

This MR adds a description (optional) field allowing users to better describe the use of a given key.

This field is available at the Project and Group Ci::Variable level.

Screenshots or screen recordings

How to set up and validate locally

1. Generate a PAT http://gdk.test:3000/-/profile/personal_access_tokens
2. Create a new variable with a description

curl --request POST --header "PRIVATE-TOKEN: <PERSONAL_ACCESS_TOKEN>" "http://gdk.test:3000/api/v4/projects/<PROJECT_ID>/variables" --form "key=NEW_VARIABLE" --form "value=new value" --form "description=description"

1. Verify the variable has been created correctly

REST API

curl --header "PRIVATE-TOKEN: <PERSONAL_ACCESS_TOKEN>" "http://gdk.test:3000/api/v4/projects/<PROJECT_ID>/variables/NEW_VARIABLE"

GraphQL API

query {
 project(fullPath: "namespace1/project-1") {
   ciVariables {
     nodes {
       key
       value
       description
     }
   }
 }
}

MR acceptance checklist

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