Lambda - Add related identifier to DMP

Build out the stubbed Lambda function that adds a related identifier to a DMP

[ ] Define how we want to record provenance for the related identifier (will have implications on future calls to PUT /dmps/[:dmp_id]
[ ] Call Lambda that validates the incoming JSON
[ ] Fetch the current version of the DMP from the DynamoDB
[ ] Code logic that adds the related identifier
[ ] Add the item to the DynamoDB
[ ] Call Lambda that handles EZID communication
[ ] Call Lambda that handles versioning
[ ] Return the JSON
[ ] Build tests

CDLUC3 / dmphub