Add metric for basefee

[parithosh]

The EL blocks contain a field called basefee. This basefee fluctuation is something we'd like to track on shadow forks. It'd be great if we can add this and update the dashboard to visualize the fluctuations.

[samcm]

Shouldn't be too hard, we've actually already got the metric created but its not being set since it didn't look correct 👍

https://github.com/samcm/ethereum-metrics-exporter/blob/main/pkg/exporter/execution/jobs/block.go#L230

Will have a look at it early next week 🎉