Add json schema file and refer to it from all task.json files

[AArnott]

Fixes #1854

[bryanmacfarlane]

Is there a mechanism to point to a published schema via http url? The reason I ask is this repo is our in the box tasks. Other task authors want to do the same and this will lead to this being the source of truth and everyone copying everywhere.

https://github.com/Microsoft/vsts-task-lib is probably the proper home for the definitive source of truth on the schema. It's our repo for all task authors.

[AArnott]

@bryanmacfarlane: yes, a URL works fine for a $schema field in a json file. And GitHub offers raw content over HTTP so I've done this many times. I'll send a PR to vsts-task-lib as you suggest.

[AArnott]

PR filed: microsoft/azure-pipelines-task-lib#146

When that's approved, I will revise this PR to consume that json file from the task.json files here.