GraphQL as a modern access to jBPM process engine

Summary
Design and implement GraphQL transport layer for jBPM services.

Initially GraphQL transport layer is going to offer following capabilities:

• subset of current capabilities of jBPM services
• additional relevant queries that can enhance capabilities of jBPM service layer
• support easy way of consuming data from engine via GraphQL in UI for building reactive business apps

Goals
Implement a GrahpQL support for KIE Server similar to what there is for REST.
Implement basic set of queries and mutations, do not copy everything from REST API and leverage it weaknesses in GraphQL.

Queries:

• processInstance( id, containerId)
• processInstances ( batchSize )
• processDefinition( definitionId, containerId )
• allProcessDefinitions( batchSize )
• variables( id, containerId )
• allTasks( batchSize )
• getTaskInstance( taskId )

Mutations:

• startProcesses( definitionId, containerId, correlationKey, variables, batchSize )
• abortProcessInstances( instanceIdList, containerId )
• signalProcessInstances( instanceIdList, signalName, event, containerId)

Acceptance criteria
User is able to use above queries and mutation using new GraphQL API availalable at `/.../graphql` endpoint
GrahpQl should use same authentication as KIE Server.