Add Beacon Chain explorer URLs to Chain

[iamacook]

Is your feature request related to a problem? Please describe.
In order to seeing validator values, the clients link to the relative Beacon Chain explorer. Currently this is done with a hardcoded value per chain. In order to make this dynamic, we should have the option to set and return it here.

Describe the solution you'd like
Add a new field similar to blockExplorerUriTemplate for the Beacon Chain explorer.