* Implemented more REST API endpoints.
- Added many of the endpoints, which return 501 - Not Implemented
- Created helper function to return a not implemented error
- Created a parse_pubkey function to get a PublicKey from a hex string
- Created a HTTP handler for the validator endpoints
- Started implementing validator/duties endpoint.
* Fleshed out get validator duties.
- Re-implemented the get validator duties function for the REST API
- Added an 'as_hex_string' function to FakePublicKey, beacuse it was missing.
* Fixed small caching/state bug.
* Extended to array of API inputs.
- Created function for getting arrays from GET parameters.
- Extended get validator duties function to support array of validator duties.
* Tidy API to be more consistent with recent decisions
* Addressing Paul's comments.
- Cleaning up function to get list of proposers.
- Removing unnecessary serde annotations
- Clarifying error messages
- Only accept pubkeys if they are '0x' prefixed.
* Fixed formatting with rustfmt.
