Simplified attestation subnet mapping

[AgeManning]

Overview

This is a continuation of #3623 and linked PR's and issues.

There is quite a bit of complexity in how we should map a node-id to a backbone gossipsub topic. I think a number of future PRs are required in to address each complexity in turn.

In Lighthouse, we plan to adopt this simple approach, whilst discussion can continue on potentially more complex forms of this mapping. This approach simplifies the current state of the mapping and allows us to take advantage of improved discovery searches, which is the primary motivation behind this.

[leolara]

I am closing this issue because it seems stale. Please, do not hesitate to reopen it if this is a mistake