Add index on `hostname` column of `job` table

### Description Add an index on the `hostname` column of the `job` table. ### Use case/motivation Currently, running CLI healthchecks for the scheduler or dag processor can take a long time. This is particularly relevant in a setup where hostnames are dynamically generated / rotated frequently, such as in k8s deploys. The addition of the `hostname` index would help improve the performance of those CLI-based healthchecks. There are likely other areas which would benefit from this index addition. ### Related issues The addition of this feature would narrow the implementation work required for https://github.com/apache/airflow/issues/62518, but these two issues are functionally distinct. ### Are you willing to submit a PR? - [x] Yes I am willing to submit a PR! ### Code of Conduct - [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)