Add support for `merge_group` event

[DaniPopes]

Example: https://github.com/paradigmxyz/reth/actions/runs/12432023112/job/34710581239

Error: Event merge_group is not supported by CodSpeed
Error: unknown variant merge_group, expected one of push, pull_request, workflow_dispatch, schedule, local at line 1 column 13

[art049]

Yes definitely we need to support this