Add API to get last oracle value

[ghost]

Description

Provide GraphQL API for oracles:

• Subscription of the latest changes (real time with ws)
• Get the latest value of the oracle data
• Get the oracle data at a given time

Fixes #451

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

We can use the following query to get the last updates:

subscription {
   oracleUpdate {
      services {
         uco {
            eur
         }
      }
   }
}

We can get latest oracle data with:

query {
   oracleData {
      timestamp
      services {
         uco {
            eur
         }
      }
   } 
}

To fetch for a given date

query {
   oracleData(timestamp: ....) {
      services {
         uco {
            eur
         }
      }
   } 
}

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules