Enable manual dispatch of leia test workflow

[AaronFeledy]

This gives maintainers a "Run Workflow" button to run the leia tests. It also enables a rest endpoint so could potentially trigger these tests from the core repo on every release. It gets enabled in the Github UI once this is in main.

https://docs.github.com/en/actions/managing-workflow-runs-and-deployments/managing-workflow-runs/manually-running-a-workflow