Include validator indices in attestation logs (#3393)
Include validator indices in attestation logs (#3393)

## Issue Addressed

Fixes #2967

## Proposed Changes

Collect validator indices alongside attestations when creating signed
attestations (and aggregates) for inclusion in the logs.

## Additional Info

This is my first time looking at Lighthouse source code and using Rust, so newbie feedback appreciated!



sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse