Update block proposer API

[pro-wh]

1. Can we add proposer and signers props to block details endpoint https://nexus.oasis.io/v1/consensus/blocks/{height}? Props were added to blocks list endpoint in consensus: save block metadata #646 and we need to show the same data in block details page.

2. Can we define type for entity_metadata so it is not unknown ?

expected type is

interface EntityMetadata {
  v: number
  serial: number
  name?: string
  url?: string
  email?: string
  keybase?: string
  twitter?: string
}

based on comment:
When available, it is an object with some subset of the following fields:

• v: The version of the metadata structure (always present).
• serial: The serial number of the metadata statement (always present).
• name: The name of the entity.
• url: The URL associated with the entity.
• email: The email address associated with the entity.
• keybase: Tne entity's keybase.io handle.
• twitter: The twitter handle associated with the entity.

[ptrus]

Can we add proposer and signers props to block details endpoint

This was done.

Can we define type for entity_metadata so it is not unknown ?

We could, but we don't enforce the type in the DB level so I'm not sure if we should do it in the API.