## Issue Addressed
Which issue # does this PR address?
https://github.com/sigp/lighthouse/issues/3669
## Proposed Changes
Please list or describe the changes introduced by this PR.
- A new API to fetch fork choice data, as specified [here](https://github.com/ethereum/beacon-APIs/pull/232)
- A new integration test to test the new API
## Additional Info
Please provide any additional information. For example, future considerations
or information useful for reviewers.
- `extra_data` field specified in the beacon-API spec is not implemented, please let me know if I should instead.
Co-authored-by: Michael Sproul <micsproul@gmail.com>