Adding Ephemery to Lodestar: generating genesis.ssz

tldr;
Generating ephemery genesis inside Lodestar.

ephemery package

Native genesis

The ephemery package needs to calculate the latest ephemery iteration and output a file genesis.ssz

• TBC whether this should be when yarn run build is run (a) at root level, (b) at package level, or when a function generateGenesis() is executed

The ephemery config should grab this output file if it is available.
If it is not available, then it should default to the current URLs.

Since this code is written as its own package inside the Lodestar repo, it may also be used by other packages/projects wishing to generate the ephemery genesis as a dependency.

What else?

Reviewing ephemery genesis script to understand what else needs to be written as output to Lodestar at packages/ephemery/lib, ensuring alignment with ephemery specs

Additional reading

• https://github.com/protolambda/eth2-testnet-genesis