[New Data Source]: Linear

### Background [Linear](https://linear.app) is a Jira-like project management tool with plenty of integrations, including many analytics integrations - it would be great to add Grafana to the list. ### Use case We would like to visualize stats like backlog growth over time; issue completion rate; project statistics; ... ### Contribution - [ ] Are you looking to create the plugin? - [ ] Are you affiliated with the project/product the data source integrates with? - [ ] Does the plugin integrate with a commercial product?