GET /highWatermark endpoint

[siladu]

Part of #696

GET http://127.0.0.1:9000/api/v1/eth2/highWatermark
returns either:

{
   "epoch": 1,
   "slot": 2
}

or empty json object when no high watermark set.

Allow epoch or slot to be null and respond with null string for that value (manual updates to DB can lead to this, even if high-watermark subcommand prevents it).

No automated tests for the endpoint in this PR as it will be tested as part of the overall subcommand acceptance test

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

Testing

• I thought about testing these changes in a realistic/non-local environment.