Write attestation schedule for epoch to console when received

[chrishobcroft]

Description

As an operator of validating infrastructure, I would like to be able to see when my Validators are scheduled to attest / propose at the start of an epoch. This allows me to better time any restarts / updates to avoid missing attestions / proposals.

For example, prysm provides:

a) a full schedule of which slots active validators have been selected to attest to / propose - provided at the beginning of the epoch b) a countdown timer until next scheduled activity - provided after the previous scheduled activity (e.g. 68 seconds until next scheduled activity... 56 seconds until next scheduled activity... 44 seconds until next scheduled activity and so on).

Acceptance Criteria

When I can observe scheduled attestations / proposals written to the console at the beginning of the epoch.

Additional Information

This is a feature I use heavily when running Validators on prysm.