Allow alternative block explorer URL's for mainnet

Description:
Currently you can’t edit the Block explorer URL for mainnet as you can for other chains/networks, which makes etherscan the only available option.
I would like to use the beaconcha.in explorer as my default as it is for my other networks.

Purpose:
Allow users to choose their prefered explorer.

Extension/Mobile/Both:
Both

Images/Attacments:
image