Multi-select inputs in pipeline UI
**Problem**
GitLab CI/CD pipeline inputs with dropdown options currently only allow selecting a single value. Users need the ability to select multiple values from the dropdown options.
**Desired Feature**
Users want to select multiple options which would be combined into a single string type value using a delimiter.
**Use Cases**:
* Restarting services on multiple instances
* Building multiple Docker images in one pipeline
* Running tests with multiple tag combinations
* Any scenario requiring operations across multiple targets
issue