Workflow engine foundations

[antho1404]

Basic implementation of workflow.
For now, workflows are limited to only event and a single task (final implementation will be event or result and multiple tasks)

Testing:

service.json

{"sid":"test-workflow","name":"Test workflow","tasks":[{"key":"taskX","inputs":[{"key":"foo","type":"String","object":[]},{"key":"bar","type":"String","object":[]}],"outputs":[{"key":"res","type":"Any","object":[]}]}],"events":[{"key":"eventX","data":[{"key":"foo","type":"String","object":[]},{"key":"bar","type":"String","object":[]}]}],"dependencies":[],"source":"QmQvRzJPFDhyBGK2rQP5mAeMrgp1XsTB8WYK2c7FHvyAB8"}

grpcurl -plaintext -d "$(cat service.json)" localhost:50052 api.Service/Create
grpcurl -plaintext -d "{\"serviceHash\":\"C8Go4Dmuc6hJaxQZzii6Jcp8ym2fhynULabKf4VowpCx\"}" localhost:50052 api.Instance/Create
grpcurl -plaintext -d "{\"filter\": {\"statuses\": [\"Completed\"]}}" localhost:50052 api.
Execution/Stream

🎉 you have new executions created because of the event from the service that is triggered by this workflow

[krhubert]

eventStream is used only in start method so it can be removed here

[antho1404]

This is used to check that the workflow engine is not started already and we should keep it in order to close the listener when we stop the workflow engine (not yet implemented)