Add CI_PROJECT_TOPICS predefined CI/CD variable

Proposal

Many predefined variables are available in the GitLab CI context. However, one useful variable is currently missing: a variable that exposes the list of topics assigned to a project. This can be very helpful for implementing policies or workflows based on project topics.

Currently, there is no predefined variable like CI_PROJECT_TOPICS. This issue proposes adding a new predefined variable, CI_PROJECT_TOPICS, which would provide a comma-separated list of the project's topics to CI jobs. This would enable more flexible and topic-aware CI/CD pipelines.