add a Create a task operation

accepts: document, documents
produces: document, documents

It creates a task attached to the input document, for the principals list taken from execution context, and accepts all parameters that we usually put on tasks (due date, directive, comment).
Two other parameters are also accepted: operation chain id to launch when validating and rejecting the task. If they're empty, no operation chain is launched.

Features to implement on the Nuxeo server side are described in NXP-5209.