[tk:backend] Accept new contribution of type "APIEvent"

[ascariandrea]

The issue #803 will let the extension send a new APIEvent to the tk:backend as contribution that needs to be properly handled.

Proposed Solution

• extend the tk:backend schema with a new collection called apiRequests
  • define indexes for id (unique), experimentId, publicKey
• extend the current handler for POST /v2/events to accept the new APIEvent contribution and store it in the proper collection apiRequests