Track oracle transactions

[hieronx]

• Track oracleFeed.Fed events
• Create OracleTransaction entities

type OracleTransaction @entity {
  id: ID! # extrinsic hash
  timestamp: Date!
  key: String!
  value: BigInt!
}

[hieronx]

Remaining TODOs:

• Add feeder column
• Fix parsing of ISIN => 0x55533931323739374c443730 => \\\US912797LD70 => US912797LD70