I can query for variable names across reports

In order to provide a list of possible variables all reports on the dashboard, we need a new endpoint that returns the available variables and their types for the given report Ids. 

ATs:

• An endpoint exists that returns the variable names and types for the given report Ids
• API Docs are updated

API Proposal:

POST /api/variables/reports

Request Payload:
{
    "reportIds": ["reportId1", "reportId2", ...]
    // In the normal Variable names endpoint there is a namePrefix option, but we don't use this anywhere
}

Response:
[{
  name: 'name1',
  type: 'String'
}, {
  name: 'name2',
  type: 'Boolean'
}]