Implement publishEvents resolver

[c-w]

This pull request implements the publishEvents resolver in MessageSchema mentioned in project-fortis-spark#17 which is the same as the client-piece mentioned in project-fortis-spark#30

The implementation is straight forward:

1. A user uploads a file with custom events via the project-fortis-interfaces frontend
2. The frontend calls the publishEvents mutation
3. A resolvers puts the events onto an EventHub
4. From here, the project-fortis-spark will pick up the events and push them through the analysis pipeline

I've also set up a test EventHub for everyone to play around with the implementation. Use the following environment settings:

PUBLISH_EVENTS_EVENTHUB_CONNECTION_STRING='...from azure dashboard...'
PUBLISH_EVENTS_EVENTHUB_PATH='customevents'
PUBLISH_EVENTS_EVENTHUB_PARTITION='$Default'

Sample mutation for graphiql.

mutation {
  publishEvents(input:{messages:[
    {RowKey:"a",created_at:"now",message:"message",language:"llcc",
     featureCollection:{type:"bla",features:[
      {type:"foo", coordinates:[1, 2, 3]}
    ]}},
    {RowKey:"b",created_at:"now",message:"message",language:"llcc",
     featureCollection:{type:"bla",features:[
      {type:"foo", coordinates:[1, 2, 3]}
    ]}},
    {RowKey:"c",created_at:"now",message:"message",language:"llcc",
     featureCollection:{type:"bla",features:[
      {type:"foo", coordinates:[1, 2, 3]}
    ]}}
  ]})
}

Here you can see messages being sent to the EventHub from GraphQL:

[erikschlegel]

LGTM