Add ability to run EF spec tests

[mpaulucci]

Language agnostic consensus spec tests can be downloaded from https://github.com/ethereum/consensus-spec-tests/releases . We can start with the minimal preset

Here is the information about how to interpret them: https://github.com/ethereum/consensus-specs/tree/dev/tests/formats

I would start by adding support for the SSZ static types: https://github.com/ethereum/consensus-specs/blob/dev/tests/formats/ssz_static/core.md which serializes/deserializes ssz files.

Here is an example of how Lighthouse does it: https://github.com/sigp/lighthouse/blob/stable/testing/ef_tests/src/cases/ssz_static.rs