[REST API] Return a processBusinessKey field for tasks

**Problem**

When retrieving a task from the REST API, we sometimes want to easily associate this task with the business object related to the process the task is associated to. This means retrieving the "businessKey" value associated to the task's process instance.

Today, you must perform two API calls to retrieve this "businessKey" value:

1. Retrieve the task with the `query/tasks` or `runtime/tasks` API endpoint
2. Retrieve the businessKey using the `processInstanceId` value returned in 1 and the `runtime/process-instances` API endpoint

It'd be easier if we could do this with one API call only.

**Potential solution**

Return a `processBusinessKey` field when returning task objects, containing the value of the `businessKey` field of the task's process instance.

This would impact the following API endpoints:

- runtime/tasks
- query/tasks
- history/historic-task-instances
- query/historic-task-instances