Spec. for REST API (#455)
Spec. for REST API (#455)

* A first run at fleshing full REST API spec.
 - Added a new REST OpenAPI YAML specification to the docs folder, starting from the minimal validator spec.
 - Added a bunch of additional endpoints, including network information and beacon chain information.
 - Current yaml file has not been checked for syntax or any correctness.

* Fixed REST OpenAPI Spec.
 - Updated spelling mistakes, indentation, and incorrect fields.

* Added block_discovery endpoint to REST API spec.

* Added /node/stats endpoint
 - /node/stats endpoint provides information about the running process
 - Added some extra TODOs as reminders.

* Added missing Attestations to REST spec.
 - Added ability to get attestations and pending attestations from chain data.
 - Moved the Attestaion object into its own schema, with reference.

sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse