(feat): Add Median Aggregator to OCR3 capability

[justinkaseman]

Description

Add Median Aggregator, that takes in the output of an Action:

type CapabilityResponse struct {
    Value *values.Map
}

and returns a single signed report with:

Reports: 
[
  {
  FeedID: []byte
  RawReport:  nil,
  Price:      big.Int,
  Timestamp:  time.Time.Unix(),
  RemappedID: string
  },
...
]

Caveats

• Output structure lifted from what will currently work with Chain Writer and the current Feeds Consumer contract, with no modifications.
• Input should have observations data as bytes, but this is not enforced. Open to ideas here:
  • More strict? Error on receiving non-numerical bytes?
  • Less strict by supporting other types such as big.Int.FromString()
• Currently supports only a single value. Should be straight forward to extend by providing multiple ValueKeys in config. and then returning multiple reports
• This will need to be merged for a proper integration test within the Chainlink repo