Add description field to the CI/CD variables setting.

Currently, we can define multiple CI/CD variables on the project/group level, and for that we need to define the type, variable name and value, few settings like protected, masked or an environment scope.

However there is no way to define what this variable is for. We've found it very problematic while working on https://gitlab.com/gitlab-com/gl-security/secops/operations/-/issues/757. There weveral variables that we needed to update and no one have good enough knowledge of the project and history behind the variables. Answers for questions like:

• What this variable is used for?
• Who created it?
• Who knows where else it is used (e.g. a token that could be used in multiple other projects)?
• Who should be immediately informed when it's changed?
• etc

All of these information were important, but we had no clue where to start and who could answer them.

So the proposal is to add a Description textarea field, where such information like the purpose or the ownership of the variable could be described. We could consider making such field optional, required or make it configurable on the group/project level.