Update slashing protection interchange to support EIP-3076

[michaelsproul]

Description

EIP-3076 is an updated version of the slashing protection interchange spec that was first introduced to Nimbus in #1643. As far as I can tell, that PR implemented v3 of the spec, and that remains the version implemented by Nimbus today.

As part of EIP-3076, it would be great to have feedback on the tests, and know that Nimbus is conforming to them. There are currently a few points open for discussion about whether certain interchange files should be considered invalid, see: https://ethereum-magicians.org/t/eip-3076-validator-client-interchange-format-slashing-protection/4883/2

Documentation on how to use Nimbus' import/export would also be very desirable. I ran make ncli_db, but it didn't seem to build a binary with import/export support (according to ncli_db --help).