Update Shutter API to include ETDs

[ylembachar]

• Update Shutter API to include ETDs with only an allowed list of contracts at first and the possibility to add contracts
• Add swagger docs
• Update documentation with examples

[konradkonrad]

https://github.com/shutter-network/shutter-api/tree/event_endpoints tracks my development on this.
TODO/DONE

• ETD compiler endpoint and simplistic standalone binary
• unit tests for compiler endpoint
• registration endpoint
• unit tests for registration endpoint
• swagger docs
• examples
• Make sure, keypers handle colliding identities correctly (between time based and event based) ==> keyper should add hash of event trigger definition when computing identity in eventtriggerregisteredprocessor.go.